diff --git a/src/api/messages/getMessages.js b/src/api/messages/getMessages.js
index 3e0ebc93589..7e5d114b03a 100644
--- a/src/api/messages/getMessages.js
+++ b/src/api/messages/getMessages.js
@@ -1,7 +1,7 @@
 /* @flow strict-local */
 import type { Auth, ApiResponseSuccess } from '../transportTypes';
 import type { Identity } from '../../types';
-import type { Message, Narrow } from '../apiTypes';
+import type { Message, ApiNarrow } from '../apiTypes';
 import type { Reaction } from '../modelTypes';
 import { apiGet } from '../apiFetch';
 import { identityOfAuth } from '../../account/accountMisc';
@@ -86,7 +86,7 @@ const migrateResponse = (response, identity: Identity) => {
 export default async (
   auth: Auth,
   args: {|
-    narrow: Narrow,
+    narrow: ApiNarrow,
     anchor: number,
     numBefore: number,
     numAfter: number,
diff --git a/src/api/modelTypes.js b/src/api/modelTypes.js
index 638c6195e3d..7e9076882e4 100644
--- a/src/api/modelTypes.js
+++ b/src/api/modelTypes.js
@@ -312,7 +312,13 @@ export type NarrowElement = $ReadOnly<{|
   operator: NarrowOperator,
 |}>;
 
-export type Narrow = $ReadOnlyArray<NarrowElement>;
+/**
+ * A narrow, in the form used in the Zulip API at get-messages.
+ *
+ * See also `Narrow` in the non-API app code, which describes how we
+ * represent narrows within the app.
+ */
+export type ApiNarrow = $ReadOnlyArray<NarrowElement>;
 
 //
 //
diff --git a/src/api/registerForEvents.js b/src/api/registerForEvents.js
index 4f23f32b1f7..655849892d0 100644
--- a/src/api/registerForEvents.js
+++ b/src/api/registerForEvents.js
@@ -1,7 +1,7 @@
 /* @flow strict-local */
 import type { RawInitialData, InitialData } from './initialDataTypes';
 import type { Auth } from './transportTypes';
-import type { Narrow } from './apiTypes';
+import type { ApiNarrow } from './apiTypes';
 import type { CrossRealmBot, User } from './modelTypes';
 import { apiPost } from './apiFetch';
 import { AvatarURL } from '../utils/avatar';
@@ -13,7 +13,7 @@ type RegisterForEventsParams = {|
   event_types?: string[],
   fetch_event_types?: string[],
   include_subscribers?: boolean,
-  narrow?: Narrow,
+  narrow?: ApiNarrow,
   queue_lifespan_secs?: number,
   client_capabilities?: {|
     notification_settings_null: boolean,
diff --git a/src/caughtup/__tests__/caughtUpReducer-test.js b/src/caughtup/__tests__/caughtUpReducer-test.js
index 83a54d811cb..dd07f4ba47b 100644
--- a/src/caughtup/__tests__/caughtUpReducer-test.js
+++ b/src/caughtup/__tests__/caughtUpReducer-test.js
@@ -10,6 +10,7 @@ import {
   HOME_NARROW_STR,
   ALL_PRIVATE_NARROW,
   ALL_PRIVATE_NARROW_STR,
+  SEARCH_NARROW,
 } from '../../utils/narrow';
 
 describe('caughtUpReducer', () => {
@@ -42,7 +43,7 @@ describe('caughtUpReducer', () => {
 
       const action = deepFreeze({
         ...eg.action.message_fetch_start,
-        narrow: [{ operator: 'search', operand: 'some query' }],
+        narrow: SEARCH_NARROW('some query'),
       });
 
       const newState = caughtUpReducer(initialState, action);
@@ -126,7 +127,7 @@ describe('caughtUpReducer', () => {
 
       const action = deepFreeze({
         ...eg.action.message_fetch_complete,
-        narrow: [{ operator: 'search', operand: 'some query' }],
+        narrow: SEARCH_NARROW('some query'),
       });
 
       const newState = caughtUpReducer(initialState, action);
diff --git a/src/chat/MarkUnreadButton.js b/src/chat/MarkUnreadButton.js
index d5f6abe15ed..bdf6a04fb3a 100644
--- a/src/chat/MarkUnreadButton.js
+++ b/src/chat/MarkUnreadButton.js
@@ -8,7 +8,13 @@ import { connect } from '../react-redux';
 import { ZulipButton } from '../common';
 import * as api from '../api';
 import { getAuth, getStreams } from '../selectors';
-import { isHomeNarrow, isStreamNarrow, isTopicNarrow } from '../utils/narrow';
+import {
+  isHomeNarrow,
+  isStreamNarrow,
+  isTopicNarrow,
+  streamNameOfNarrow,
+  topicOfNarrow,
+} from '../utils/narrow';
 
 const styles = createStyleSheet({
   button: {
@@ -34,7 +40,8 @@ class MarkUnreadButton extends PureComponent<Props> {
 
   markStreamAsRead = () => {
     const { auth, narrow, streams } = this.props;
-    const stream = streams.find(s => s.name === narrow[0].operand);
+    const streamName = streamNameOfNarrow(narrow);
+    const stream = streams.find(s => s.name === streamName);
     if (stream) {
       api.markStreamAsRead(auth, stream.stream_id);
     }
@@ -42,9 +49,10 @@ class MarkUnreadButton extends PureComponent<Props> {
 
   markTopicAsRead = () => {
     const { auth, narrow, streams } = this.props;
-    const stream = streams.find(s => s.name === narrow[0].operand);
+    const streamName = streamNameOfNarrow(narrow);
+    const stream = streams.find(s => s.name === streamName);
     if (stream) {
-      api.markTopicAsRead(auth, stream.stream_id, narrow[1].operand);
+      api.markTopicAsRead(auth, stream.stream_id, topicOfNarrow(narrow));
     }
   };
 
diff --git a/src/chat/__tests__/fetchingReducer-test.js b/src/chat/__tests__/fetchingReducer-test.js
index 64093bd808b..85294e84f73 100644
--- a/src/chat/__tests__/fetchingReducer-test.js
+++ b/src/chat/__tests__/fetchingReducer-test.js
@@ -3,7 +3,13 @@ import deepFreeze from 'deep-freeze';
 
 import * as eg from '../../__tests__/lib/exampleData';
 import fetchingReducer from '../fetchingReducer';
-import { HOME_NARROW, HOME_NARROW_STR, streamNarrow, keyFromNarrow } from '../../utils/narrow';
+import {
+  HOME_NARROW,
+  HOME_NARROW_STR,
+  streamNarrow,
+  keyFromNarrow,
+  SEARCH_NARROW,
+} from '../../utils/narrow';
 import { MESSAGE_FETCH_START, MESSAGE_FETCH_ERROR } from '../../actionConstants';
 import { DEFAULT_FETCHING } from '../fetchingSelectors';
 
@@ -16,7 +22,7 @@ describe('fetchingReducer', () => {
 
       const action = deepFreeze({
         type: MESSAGE_FETCH_START,
-        narrow: [],
+        narrow: HOME_NARROW,
         numBefore: 10,
         numAfter: 10,
       });
@@ -62,7 +68,7 @@ describe('fetchingReducer', () => {
 
       const action = deepFreeze({
         ...eg.action.message_fetch_start,
-        narrow: [{ operator: 'search', operand: 'some query' }],
+        narrow: SEARCH_NARROW('some query'),
       });
 
       const newState = fetchingReducer(initialState, action);
@@ -108,7 +114,7 @@ describe('fetchingReducer', () => {
 
       const action = {
         ...eg.action.message_fetch_complete,
-        narrow: [],
+        narrow: HOME_NARROW,
         numBefore: 10,
         numAfter: 0,
       };
@@ -130,7 +136,7 @@ describe('fetchingReducer', () => {
 
     const action = deepFreeze({
       ...eg.action.message_fetch_complete,
-      narrow: [{ operator: 'search', operand: 'some query' }],
+      narrow: SEARCH_NARROW('some query'),
     });
 
     const newState = fetchingReducer(initialState, action);
diff --git a/src/chat/__tests__/narrowsReducer-test.js b/src/chat/__tests__/narrowsReducer-test.js
index f661b6dd717..bd7d05b0fc3 100644
--- a/src/chat/__tests__/narrowsReducer-test.js
+++ b/src/chat/__tests__/narrowsReducer-test.js
@@ -13,6 +13,7 @@ import {
   topicNarrow,
   STARRED_NARROW_STR,
   keyFromNarrow,
+  SEARCH_NARROW,
 } from '../../utils/narrow';
 import {
   MESSAGE_FETCH_ERROR,
@@ -324,7 +325,7 @@ describe('narrowsReducer', () => {
 
       const action = deepFreeze({
         ...eg.action.message_fetch_start,
-        narrow: [{ operator: 'search', operand: 'some query' }],
+        narrow: SEARCH_NARROW('some query'),
       });
 
       const newState = narrowsReducer(initialState, action);
@@ -396,7 +397,7 @@ describe('narrowsReducer', () => {
       const action = deepFreeze({
         type: MESSAGE_FETCH_COMPLETE,
         anchor: 2,
-        narrow: [],
+        narrow: HOME_NARROW,
         messages: [
           eg.streamMessage({ id: 2 }),
           eg.streamMessage({ id: 3 }),
@@ -424,7 +425,7 @@ describe('narrowsReducer', () => {
       const action = deepFreeze({
         type: MESSAGE_FETCH_COMPLETE,
         anchor: 2,
-        narrow: [],
+        narrow: HOME_NARROW,
         messages: [
           eg.streamMessage({ id: 3, timestamp: 2 }),
           eg.streamMessage({ id: 4, timestamp: 1 }),
@@ -451,7 +452,7 @@ describe('narrowsReducer', () => {
       const action = deepFreeze({
         anchor: FIRST_UNREAD_ANCHOR,
         type: MESSAGE_FETCH_COMPLETE,
-        narrow: [],
+        narrow: HOME_NARROW,
         messages: [
           eg.streamMessage({ id: 3, timestamp: 2 }),
           eg.streamMessage({ id: 4, timestamp: 1 }),
@@ -477,7 +478,7 @@ describe('narrowsReducer', () => {
       const action = deepFreeze({
         anchor: LAST_MESSAGE_ANCHOR,
         type: MESSAGE_FETCH_COMPLETE,
-        narrow: [],
+        narrow: HOME_NARROW,
         messages: [
           eg.streamMessage({ id: 3, timestamp: 2 }),
           eg.streamMessage({ id: 4, timestamp: 1 }),
@@ -502,7 +503,7 @@ describe('narrowsReducer', () => {
 
       const action = deepFreeze({
         ...eg.action.message_fetch_complete,
-        narrow: [{ operator: 'search', operand: 'some query' }],
+        narrow: SEARCH_NARROW('some query'),
       });
 
       const newState = narrowsReducer(initialState, action);
diff --git a/src/chat/narrowsSelectors.js b/src/chat/narrowsSelectors.js
index dc1ffb53660..b62d402011f 100644
--- a/src/chat/narrowsSelectors.js
+++ b/src/chat/narrowsSelectors.js
@@ -29,6 +29,7 @@ import {
   isMessageInNarrow,
   caseNarrowDefault,
   keyFromNarrow,
+  streamNameOfNarrow,
 } from '../utils/narrow';
 import { shouldBeMuted } from '../utils/message';
 import { NULL_ARRAY, NULL_SUBSCRIPTION } from '../nullObjects';
@@ -125,13 +126,14 @@ export const getStreamInNarrow: Selector<Subscription | {| ...Stream, in_home_vi
     if (!isStreamOrTopicNarrow(narrow)) {
       return NULL_SUBSCRIPTION;
     }
+    const streamName = streamNameOfNarrow(narrow);
 
-    const subscription = subscriptions.find(x => x.name === narrow[0].operand);
+    const subscription = subscriptions.find(x => x.name === streamName);
     if (subscription) {
       return subscription;
     }
 
-    const stream = streams.find(x => x.name === narrow[0].operand);
+    const stream = streams.find(x => x.name === streamName);
     if (stream) {
       return {
         ...stream,
diff --git a/src/compose/ComposeBox.js b/src/compose/ComposeBox.js
index 7945354cc76..6fcbc82cdb8 100644
--- a/src/compose/ComposeBox.js
+++ b/src/compose/ComposeBox.js
@@ -33,7 +33,12 @@ import * as api from '../api';
 import { FloatingActionButton, Input } from '../common';
 import { showErrorAlert } from '../utils/info';
 import { IconDone, IconSend } from '../common/Icons';
-import { isStreamNarrow, isStreamOrTopicNarrow, topicNarrow } from '../utils/narrow';
+import {
+  isStreamNarrow,
+  isStreamOrTopicNarrow,
+  streamNameOfNarrow,
+  topicNarrow,
+} from '../utils/narrow';
 import ComposeMenu from './ComposeMenu';
 import getComposeInputPlaceholder from './getComposeInputPlaceholder';
 import NotSubscribed from '../message/NotSubscribed';
@@ -307,8 +312,12 @@ class ComposeBox extends PureComponent<Props, State> {
 
   getDestinationNarrow = (): Narrow => {
     const { narrow } = this.props;
-    const topic = this.state.topic.trim();
-    return isStreamNarrow(narrow) ? topicNarrow(narrow[0].operand, topic || '(no topic)') : narrow;
+    if (isStreamNarrow(narrow)) {
+      const streamName = streamNameOfNarrow(narrow);
+      const topic = this.state.topic.trim();
+      return topicNarrow(streamName, topic || '(no topic)');
+    }
+    return narrow;
   };
 
   handleSend = () => {
diff --git a/src/compose/getComposeInputPlaceholder.js b/src/compose/getComposeInputPlaceholder.js
index ec24a0d1f55..ff7a5563633 100644
--- a/src/compose/getComposeInputPlaceholder.js
+++ b/src/compose/getComposeInputPlaceholder.js
@@ -1,42 +1,40 @@
 /* @flow strict-local */
 import type { Narrow, UserOrBot, LocalizableText } from '../types';
-import { isStreamNarrow, isTopicNarrow, isGroupPmNarrow, is1to1PmNarrow } from '../utils/narrow';
+import { caseNarrowDefault } from '../utils/narrow';
 
 export default (
   narrow: Narrow,
   ownEmail: string,
   usersByEmail: Map<string, UserOrBot>,
-): LocalizableText => {
-  if (isGroupPmNarrow(narrow)) {
-    return { text: 'Message group' };
-  }
+): LocalizableText =>
+  caseNarrowDefault(
+    narrow,
+    {
+      pm: emails => {
+        if (emails.length > 1) {
+          return { text: 'Message group' };
+        }
+        const email = emails[0];
 
-  if (is1to1PmNarrow(narrow)) {
-    if (ownEmail && narrow[0].operand === ownEmail) {
-      return { text: 'Jot down something' };
-    }
+        if (email === ownEmail) {
+          return { text: 'Jot down something' };
+        }
 
-    if (!usersByEmail) {
-      return { text: 'Type a message' };
-    }
+        const user = usersByEmail.get(email);
+        if (!user) {
+          return { text: 'Type a message' };
+        }
 
-    const user = usersByEmail.get(narrow[0].operand) || {};
-    return {
-      text: 'Message {recipient}',
-      values: { recipient: `@${user.full_name}` },
-    };
-  }
-
-  if (isStreamNarrow(narrow)) {
-    return {
-      text: 'Message {recipient}',
-      values: { recipient: `#${narrow[0].operand}` },
-    };
-  }
-
-  if (isTopicNarrow(narrow)) {
-    return { text: 'Reply' };
-  }
-
-  return { text: 'Type a message' };
-};
+        return {
+          text: 'Message {recipient}',
+          values: { recipient: `@${user.full_name}` },
+        };
+      },
+      stream: name => ({
+        text: 'Message {recipient}',
+        values: { recipient: `#${name}` },
+      }),
+      topic: () => ({ text: 'Reply' }),
+    },
+    () => ({ text: 'Type a message' }),
+  );
diff --git a/src/message/__tests__/messageActionSheet-test.js b/src/message/__tests__/messageActionSheet-test.js
index 5372a61585f..d6f6b9f463b 100644
--- a/src/message/__tests__/messageActionSheet-test.js
+++ b/src/message/__tests__/messageActionSheet-test.js
@@ -1,5 +1,6 @@
 // @flow strict-local
 import deepFreeze from 'deep-freeze';
+import { HOME_NARROW } from '../../utils/narrow';
 
 import * as eg from '../../__tests__/lib/exampleData';
 import { constructMessageActionButtons, constructHeaderActionButtons } from '../messageActionSheet';
@@ -18,7 +19,7 @@ const baseBackgroundData = deepFreeze({
 });
 
 describe('constructActionButtons', () => {
-  const narrow = deepFreeze([]);
+  const narrow = deepFreeze(HOME_NARROW);
 
   test('show star message option if message is not starred', () => {
     const message = eg.streamMessage();
@@ -53,7 +54,7 @@ describe('constructActionButtons', () => {
 });
 
 describe('constructHeaderActionButtons', () => {
-  const narrow = deepFreeze([]);
+  const narrow = deepFreeze(HOME_NARROW);
 
   test('show Unmute topic option if topic is muted', () => {
     const mute = deepFreeze([['electron issues', 'issue #556']]);
diff --git a/src/message/__tests__/renderMessages-test.js b/src/message/__tests__/renderMessages-test.js
index 110111f4f7f..079c14db968 100644
--- a/src/message/__tests__/renderMessages-test.js
+++ b/src/message/__tests__/renderMessages-test.js
@@ -4,7 +4,7 @@ import { HOME_NARROW } from '../../utils/narrow';
 import renderMessages from '../renderMessages';
 
 describe('renderMessages', () => {
-  const narrow = deepFreeze([]);
+  const narrow = deepFreeze(HOME_NARROW);
 
   test('empty messages results in a single empty section', () => {
     const messageList = renderMessages([], HOME_NARROW);
diff --git a/src/message/fetchActions.js b/src/message/fetchActions.js
index 7f1c2bcc410..e90c4196513 100644
--- a/src/message/fetchActions.js
+++ b/src/message/fetchActions.js
@@ -24,7 +24,7 @@ import {
   MESSAGE_FETCH_COMPLETE,
 } from '../actionConstants';
 import { FIRST_UNREAD_ANCHOR, LAST_MESSAGE_ANCHOR } from '../anchor';
-import { ALL_PRIVATE_NARROW } from '../utils/narrow';
+import { ALL_PRIVATE_NARROW, apiNarrowOfNarrow } from '../utils/narrow';
 import { BackoffMachine } from '../utils/async';
 import { initNotifications } from '../notification/notificationActions';
 import { addToOutbox, sendOutbox } from '../outbox/outboxActions';
@@ -88,6 +88,7 @@ export const fetchMessages = (fetchArgs: {|
   try {
     const { messages, found_newest, found_oldest } = await api.getMessages(getAuth(getState()), {
       ...fetchArgs,
+      narrow: apiNarrowOfNarrow(fetchArgs.narrow),
       useFirstUnread: fetchArgs.anchor === FIRST_UNREAD_ANCHOR, // TODO: don't use this; see #4203
     });
     dispatch(
@@ -235,7 +236,7 @@ export const fetchMessagesInNarrow = (
 const fetchPrivateMessages = () => async (dispatch: Dispatch, getState: GetState) => {
   const auth = getAuth(getState());
   const { messages, found_newest, found_oldest } = await api.getMessages(auth, {
-    narrow: ALL_PRIVATE_NARROW,
+    narrow: apiNarrowOfNarrow(ALL_PRIVATE_NARROW),
     anchor: LAST_MESSAGE_ANCHOR,
     numBefore: 100,
     numAfter: 0,
diff --git a/src/reduxTypes.js b/src/reduxTypes.js
index 5a1a42d2255..2926af269d6 100644
--- a/src/reduxTypes.js
+++ b/src/reduxTypes.js
@@ -21,7 +21,6 @@ import type {
   CrossRealmBot,
   RealmEmojiById,
   RealmFilter,
-  Narrow,
   Stream,
   StreamUnreadItem,
   Subscription,
@@ -30,7 +29,7 @@ import type {
   UserPresence,
   UserStatusMapObject,
 } from './api/apiTypes';
-
+import type { Narrow } from './utils/narrow';
 import type { SessionState } from './session/sessionReducer';
 
 export type * from './actionTypes';
diff --git a/src/search/SearchMessagesCard.js b/src/search/SearchMessagesCard.js
index f38dcc67b28..640f7278ca5 100644
--- a/src/search/SearchMessagesCard.js
+++ b/src/search/SearchMessagesCard.js
@@ -45,7 +45,7 @@ export default class SearchMessagesCard extends PureComponent<Props> {
       return <SearchEmptyState text="No results" />;
     }
 
-    const renderedMessages = renderMessages(messages, []);
+    const renderedMessages = renderMessages(messages, HOME_NARROW);
 
     return (
       <View style={styles.results}>
diff --git a/src/subscriptions/subscriptionSelectors.js b/src/subscriptions/subscriptionSelectors.js
index 4d7436867f4..deac6b6de23 100644
--- a/src/subscriptions/subscriptionSelectors.js
+++ b/src/subscriptions/subscriptionSelectors.js
@@ -2,7 +2,7 @@
 import { createSelector } from 'reselect';
 
 import type { GlobalState, Narrow, Selector, Stream, Subscription } from '../types';
-import { isStreamOrTopicNarrow } from '../utils/narrow';
+import { isStreamOrTopicNarrow, streamNameOfNarrow } from '../utils/narrow';
 import { getSubscriptions, getStreams } from '../directSelectors';
 
 /**
@@ -39,8 +39,9 @@ export const getIsActiveStreamSubscribed: Selector<boolean, Narrow> = createSele
     if (!isStreamOrTopicNarrow(narrow)) {
       return true;
     }
+    const streamName = streamNameOfNarrow(narrow);
 
-    return subscriptions.find(sub => narrow[0].operand === sub.name) !== undefined;
+    return subscriptions.find(sub => streamName === sub.name) !== undefined;
   },
 );
 
@@ -79,7 +80,9 @@ export const getIsActiveStreamAnnouncementOnly: Selector<boolean, Narrow> = crea
     if (!isStreamOrTopicNarrow(narrow)) {
       return false;
     }
-    const stream = streams.find(stream_ => narrow[0].operand === stream_.name);
+    const streamName = streamNameOfNarrow(narrow);
+
+    const stream = streams.find(stream_ => streamName === stream_.name);
     return stream ? stream.is_announcement_only : false;
   },
 );
diff --git a/src/title-buttons/ExtraNavButtonStream.js b/src/title-buttons/ExtraNavButtonStream.js
index 94ee3da0be9..be44468be5d 100644
--- a/src/title-buttons/ExtraNavButtonStream.js
+++ b/src/title-buttons/ExtraNavButtonStream.js
@@ -8,6 +8,7 @@ import { connect } from '../react-redux';
 import { getStreams } from '../selectors';
 import NavButton from '../nav/NavButton';
 import { navigateToTopicList } from '../actions';
+import { streamNameOfNarrow } from '../utils/narrow';
 
 type Props = $ReadOnly<{|
   dispatch: Dispatch,
@@ -19,7 +20,8 @@ type Props = $ReadOnly<{|
 class ExtraNavButtonStream extends PureComponent<Props> {
   handlePress = () => {
     const { narrow, streams } = this.props;
-    const stream = streams.find(x => x.name === narrow[0].operand);
+    const streamName = streamNameOfNarrow(narrow);
+    const stream = streams.find(x => x.name === streamName);
     if (stream) {
       NavigationService.dispatch(navigateToTopicList(stream.stream_id));
     }
diff --git a/src/title-buttons/ExtraNavButtonTopic.js b/src/title-buttons/ExtraNavButtonTopic.js
index 8127e58488d..a0a7e2e8903 100644
--- a/src/title-buttons/ExtraNavButtonTopic.js
+++ b/src/title-buttons/ExtraNavButtonTopic.js
@@ -5,7 +5,7 @@ import React, { PureComponent } from 'react';
 import type { Dispatch, Narrow, Stream } from '../types';
 import { connect } from '../react-redux';
 import { getStreams } from '../selectors';
-import { streamNarrow } from '../utils/narrow';
+import { streamNameOfNarrow, streamNarrow } from '../utils/narrow';
 import NavButton from '../nav/NavButton';
 import { doNarrow } from '../actions';
 
@@ -19,7 +19,8 @@ type Props = $ReadOnly<{|
 class ExtraNavButtonTopic extends PureComponent<Props> {
   handlePress = () => {
     const { dispatch, narrow, streams } = this.props;
-    const stream = streams.find(x => x.name === narrow[0].operand);
+    const streamName = streamNameOfNarrow(narrow);
+    const stream = streams.find(x => x.name === streamName);
     if (stream) {
       dispatch(doNarrow(streamNarrow(stream.name)));
     }
diff --git a/src/title-buttons/InfoNavButtonPrivate.js b/src/title-buttons/InfoNavButtonPrivate.js
index 1495200cfac..fcd70b7e076 100644
--- a/src/title-buttons/InfoNavButtonPrivate.js
+++ b/src/title-buttons/InfoNavButtonPrivate.js
@@ -8,6 +8,7 @@ import { connect } from '../react-redux';
 import NavButton from '../nav/NavButton';
 import { navigateToAccountDetails } from '../actions';
 import { getUserForEmail } from '../users/userSelectors';
+import { emailOfPm1to1Narrow } from '../utils/narrow';
 
 type SelectorProps = $ReadOnly<{|
   userId: number,
@@ -34,5 +35,5 @@ class InfoNavButtonPrivate extends PureComponent<Props> {
 }
 
 export default connect<SelectorProps, _, _>((state, props) => ({
-  userId: getUserForEmail(state, props.narrow[0].operand).user_id,
+  userId: getUserForEmail(state, emailOfPm1to1Narrow(props.narrow)).user_id,
 }))(InfoNavButtonPrivate);
diff --git a/src/title-buttons/InfoNavButtonStream.js b/src/title-buttons/InfoNavButtonStream.js
index 7f5db4d6baa..3f5426aff23 100644
--- a/src/title-buttons/InfoNavButtonStream.js
+++ b/src/title-buttons/InfoNavButtonStream.js
@@ -8,6 +8,7 @@ import { connect } from '../react-redux';
 import { getStreams } from '../selectors';
 import NavButton from '../nav/NavButton';
 import { navigateToStream } from '../actions';
+import { streamNameOfNarrow } from '../utils/narrow';
 
 type Props = $ReadOnly<{|
   dispatch: Dispatch,
@@ -19,7 +20,8 @@ type Props = $ReadOnly<{|
 class InfoNavButtonStream extends PureComponent<Props> {
   handlePress = () => {
     const { narrow, streams } = this.props;
-    const stream = streams.find(x => x.name === narrow[0].operand);
+    const streamName = streamNameOfNarrow(narrow);
+    const stream = streams.find(x => x.name === streamName);
     if (stream) {
       NavigationService.dispatch(navigateToStream(stream.stream_id));
     }
diff --git a/src/title/TitleStream.js b/src/title/TitleStream.js
index 7f4c5d5bdd2..25eefb43548 100644
--- a/src/title/TitleStream.js
+++ b/src/title/TitleStream.js
@@ -7,7 +7,7 @@ import type { Narrow, Stream, Subscription, Dispatch } from '../types';
 import styles, { createStyleSheet } from '../styles';
 import { connect } from '../react-redux';
 import StreamIcon from '../streams/StreamIcon';
-import { isTopicNarrow } from '../utils/narrow';
+import { isTopicNarrow, topicOfNarrow } from '../utils/narrow';
 import { getStreamInNarrow } from '../selectors';
 import { showToast } from '../utils/info';
 
@@ -57,11 +57,11 @@ class TitleStream extends PureComponent<Props> {
         {isTopicNarrow(narrow) && (
           <TouchableWithoutFeedback
             onLongPress={() => {
-              showToast(narrow[1].operand);
+              showToast(topicOfNarrow(narrow));
             }}
           >
             <Text style={[styles.navSubtitle, { color }]} numberOfLines={1} ellipsizeMode="tail">
-              {narrow[1].operand}
+              {topicOfNarrow(narrow)}
             </Text>
           </TouchableWithoutFeedback>
         )}
diff --git a/src/title/titleSelectors.js b/src/title/titleSelectors.js
index 2191d732487..b0db78670c6 100644
--- a/src/title/titleSelectors.js
+++ b/src/title/titleSelectors.js
@@ -1,6 +1,6 @@
 /* @flow strict-local */
 import type { Narrow, GlobalState } from '../types';
-import { isStreamOrTopicNarrow } from '../utils/narrow';
+import { isStreamOrTopicNarrow, streamNameOfNarrow } from '../utils/narrow';
 import { getSubscriptionsByName } from '../subscriptions/subscriptionSelectors';
 
 export const DEFAULT_TITLE_BACKGROUND_COLOR = 'transparent';
@@ -16,6 +16,6 @@ export const getTitleBackgroundColor = (state: GlobalState, narrow?: Narrow) =>
   if (!narrow || !isStreamOrTopicNarrow(narrow)) {
     return DEFAULT_TITLE_BACKGROUND_COLOR;
   }
-  const streamName = narrow[0].operand;
+  const streamName = streamNameOfNarrow(narrow);
   return subscriptionsByName.get(streamName)?.color ?? 'gray';
 };
diff --git a/src/topics/topicActions.js b/src/topics/topicActions.js
index 255528d9219..0b7efe74bc3 100644
--- a/src/topics/topicActions.js
+++ b/src/topics/topicActions.js
@@ -2,7 +2,7 @@
 import type { GetState, Dispatch, Narrow, Topic, Action, Outbox, Stream } from '../types';
 import * as api from '../api';
 import { INIT_TOPICS } from '../actionConstants';
-import { isStreamNarrow } from '../utils/narrow';
+import { isStreamNarrow, streamNameOfNarrow } from '../utils/narrow';
 import { getAuth, getStreams } from '../selectors';
 import { deleteOutboxMessage } from '../actions';
 import { getOutbox } from '../directSelectors';
@@ -29,9 +29,10 @@ export const fetchTopicsForStream = (narrow: Narrow) => async (
   if (!isStreamNarrow(narrow)) {
     return;
   }
+  const streamName = streamNameOfNarrow(narrow);
 
   const streams = getStreams(state);
-  const stream = streams.find(sub => narrow[0].operand === sub.name);
+  const stream = streams.find(sub => streamName === sub.name);
   if (!stream) {
     return;
   }
diff --git a/src/topics/topicSelectors.js b/src/topics/topicSelectors.js
index ab0e6268e6c..4d39beacbac 100644
--- a/src/topics/topicSelectors.js
+++ b/src/topics/topicSelectors.js
@@ -17,7 +17,7 @@ import { getMute, getStreams, getTopics, getUnreadStreams } from '../directSelec
 import { getShownMessagesForNarrow } from '../chat/narrowsSelectors';
 import { getStreamsById } from '../subscriptions/subscriptionSelectors';
 import { NULL_ARRAY } from '../nullObjects';
-import { isStreamNarrow } from '../utils/narrow';
+import { isStreamNarrow, streamNameOfNarrow } from '../utils/narrow';
 
 export const getTopicsForNarrow: Selector<string[], Narrow> = createSelector(
   (state, narrow) => narrow,
@@ -27,8 +27,9 @@ export const getTopicsForNarrow: Selector<string[], Narrow> = createSelector(
     if (!isStreamNarrow(narrow)) {
       return NULL_ARRAY;
     }
-    const stream = streams.find(x => x.name === narrow[0].operand);
+    const streamName = streamNameOfNarrow(narrow);
 
+    const stream = streams.find(x => x.name === streamName);
     if (!stream || !topics[stream.stream_id]) {
       return NULL_ARRAY;
     }
diff --git a/src/types.js b/src/types.js
index 9300b42fd1b..c0cb11b2b99 100644
--- a/src/types.js
+++ b/src/types.js
@@ -10,6 +10,7 @@ import type { PmKeyUsers } from './utils/recipient';
 export type * from './generics';
 export type * from './reduxTypes';
 export type * from './api/apiTypes';
+export type { Narrow } from './utils/narrow';
 
 export { ensureUnreachable } from './generics';
 
diff --git a/src/typing/typingSelectors.js b/src/typing/typingSelectors.js
index 447c324109e..61b928822cc 100644
--- a/src/typing/typingSelectors.js
+++ b/src/typing/typingSelectors.js
@@ -3,7 +3,7 @@ import { createSelector } from 'reselect';
 
 import type { Narrow, Selector, UserOrBot } from '../types';
 import { getTyping } from '../directSelectors';
-import { isPmNarrow } from '../utils/narrow';
+import { emailsOfPmNarrow, isPmNarrow } from '../utils/narrow';
 import { normalizeRecipientsAsUserIds } from '../utils/recipient';
 import { NULL_ARRAY, NULL_USER } from '../nullObjects';
 import { getAllUsersById, getAllUsersByEmail } from '../users/userSelectors';
@@ -18,7 +18,7 @@ export const getCurrentTypingUsers: Selector<$ReadOnlyArray<UserOrBot>, Narrow>
       return NULL_ARRAY;
     }
 
-    const recipients = narrow[0].operand.split(',').map(email => {
+    const recipients = emailsOfPmNarrow(narrow).map(email => {
       const userId = allUsersByEmail.get(email)?.user_id;
       if (userId === undefined) {
         throw new Error(`Narrow contains email '${email}' that does not map to any user.`);
diff --git a/src/unread/unreadSelectors.js b/src/unread/unreadSelectors.js
index 1b623973117..da982d94c89 100644
--- a/src/unread/unreadSelectors.js
+++ b/src/unread/unreadSelectors.js
@@ -14,13 +14,7 @@ import {
 import { getOwnEmail, getAllUsersByEmail } from '../users/userSelectors';
 import { getSubscriptionsById } from '../subscriptions/subscriptionSelectors';
 import { isTopicMuted } from '../utils/message';
-import {
-  isHomeNarrow,
-  isStreamNarrow,
-  isTopicNarrow,
-  isGroupPmNarrow,
-  is1to1PmNarrow,
-} from '../utils/narrow';
+import { caseNarrow } from '../utils/narrow';
 import { NULL_SUBSCRIPTION, NULL_USER } from '../nullObjects';
 
 /** The number of unreads in each stream, excluding muted topics, by stream ID. */
@@ -219,7 +213,8 @@ export const getUnreadByHuddlesMentionsAndPMs: Selector<number> = createSelector
 /**
  * Total number of unreads in the given narrow... mostly.
  *
- * For the @-mention narrow and search narrows, just returns 0.
+ * For the @-mention narrow, all-PMs narrow, and search narrows,
+ * just returns 0.
  *
  * For the all-messages narrow, the caveats on `getUnreadTotal` apply.
  */
@@ -244,64 +239,66 @@ export const getUnreadCountForNarrow: Selector<number, Narrow> = createSelector(
     unreadPms,
     mute,
   ) => {
-    if (isHomeNarrow(narrow)) {
-      return unreadTotal;
-    }
-
-    if (isStreamNarrow(narrow)) {
-      const stream = streams.find(s => s.name === narrow[0].operand);
+    const sumLengths = unreads => unreads.reduce((sum, x) => sum + x.unread_message_ids.length, 0);
 
-      if (!stream) {
-        return 0;
-      }
+    return caseNarrow(narrow, {
+      home: () => unreadTotal,
 
-      return unreadStreams
-        .filter(x => x.stream_id === stream.stream_id)
-        .reduce(
-          (sum, x) =>
-            sum + (isTopicMuted(stream.name, x.topic, mute) ? 0 : x.unread_message_ids.length),
-          0,
+      stream: name => {
+        const stream = streams.find(s => s.name === name);
+        if (!stream) {
+          return 0;
+        }
+        return sumLengths(
+          unreadStreams.filter(
+            x => x.stream_id === stream.stream_id && !isTopicMuted(name, x.topic, mute),
+          ),
         );
-    }
-
-    if (isTopicNarrow(narrow)) {
-      const stream = streams.find(s => s.name === narrow[0].operand);
-
-      if (!stream) {
-        return 0;
-      }
+      },
 
-      return unreadStreams
-        .filter(x => x.stream_id === stream.stream_id && x.topic === narrow[1].operand)
-        .reduce((sum, x) => sum + x.unread_message_ids.length, 0);
-    }
+      topic: (streamName, topic) => {
+        const stream = streams.find(s => s.name === streamName);
+        if (!stream) {
+          return 0;
+        }
+        return sumLengths(
+          unreadStreams.filter(x => x.stream_id === stream.stream_id && x.topic === topic),
+        );
+      },
 
-    if (isGroupPmNarrow(narrow)) {
-      const userIds = [...narrow[0].operand.split(','), ownEmail]
-        .map(email => (allUsersByEmail.get(email) || NULL_USER).user_id)
-        .sort((a, b) => a - b)
-        .join(',');
-      const unread = unreadHuddles.find(x => x.user_ids_string === userIds);
-      return unread ? unread.unread_message_ids.length : 0;
-    }
+      pm: emails => {
+        if (emails.length > 1) {
+          const userIds = [...emails, ownEmail]
+            .map(email => (allUsersByEmail.get(email) || NULL_USER).user_id)
+            .sort((a, b) => a - b)
+            .join(',');
+          const unread = unreadHuddles.find(x => x.user_ids_string === userIds);
+          return unread ? unread.unread_message_ids.length : 0;
+        } else {
+          const sender = allUsersByEmail.get(emails[0]);
+          if (!sender) {
+            return 0;
+          }
+          const unread = unreadPms.find(x => x.sender_id === sender.user_id);
+          return unread ? unread.unread_message_ids.length : 0;
+        }
+      },
 
-    if (is1to1PmNarrow(narrow)) {
-      const sender = allUsersByEmail.get(narrow[0].operand);
-      if (!sender) {
-        return 0;
-      }
-      const unread = unreadPms.find(x => x.sender_id === sender.user_id);
-      return unread ? unread.unread_message_ids.length : 0;
-    }
+      // Unread starred messages are impossible, so 0 is correct for the
+      // starred-messages narrow.
+      // TODO: fact-check that.
+      starred: () => 0,
 
-    // Unread starred messages are impossible, so 0 is correct for the
-    // starred-messages narrow.
-    // TODO: fact-check that.
+      // TODO: give a correct answer for the @-mentions narrow.
+      mentioned: () => 0,
 
-    // TODO: give a correct answer for the @-mentions narrow.
+      // For search narrows, shrug, a bogus answer of 0 is fine.
+      search: () => 0,
 
-    // For search narrows, shrug, a bogus answer of 0 is fine.
-
-    return 0;
+      // For the all-PMs narrow, a bogus answer of 0 is perfectly fine
+      // because we never use this selector for that narrow (because we
+      // don't expose it as one you can narrow to in the UI.)
+      allPrivate: () => 0,
+    });
   },
 );
diff --git a/src/users/usersActions.js b/src/users/usersActions.js
index 74e5cfbe39e..3d44f4abdd3 100644
--- a/src/users/usersActions.js
+++ b/src/users/usersActions.js
@@ -5,7 +5,7 @@ import type { Auth, Dispatch, GetState, GlobalState, Narrow } from '../types';
 import * as api from '../api';
 import { PRESENCE_RESPONSE } from '../actionConstants';
 import { getAuth, tryGetAuth, getServerVersion } from '../selectors';
-import { isPmNarrow, caseNarrowPartial } from '../utils/narrow';
+import { isPmNarrow, emailsOfPmNarrow } from '../utils/narrow';
 import { getAllUsersByEmail, getUserForId } from './userSelectors';
 import { ZulipVersion } from '../utils/zulipVersion';
 
@@ -65,9 +65,7 @@ export const sendTypingStart = (narrow: Narrow) => async (
   }
 
   const allUsersByEmail = getAllUsersByEmail(getState());
-  const recipientIds = caseNarrowPartial(narrow, {
-    pm: emails => emails,
-  }).map(email => {
+  const recipientIds = emailsOfPmNarrow(narrow).map(email => {
     const user = allUsersByEmail.get(email);
     if (!user) {
       throw new Error('unknown user');
diff --git a/src/utils/__tests__/narrow-test.js b/src/utils/__tests__/narrow-test.js
index 05f34029074..d11d60e4620 100644
--- a/src/utils/__tests__/narrow-test.js
+++ b/src/utils/__tests__/narrow-test.js
@@ -26,41 +26,29 @@ import {
   MENTIONED_NARROW,
   pm1to1NarrowFromUser,
   keyFromNarrow,
+  streamNameOfNarrow,
+  topicOfNarrow,
+  emailOfPm1to1Narrow,
 } from '../narrow';
 import type { Narrow, Message } from '../../types';
 import * as eg from '../../__tests__/lib/exampleData';
 
 describe('HOME_NARROW', () => {
-  test('produces an empty list', () => {
-    expect(HOME_NARROW).toEqual([]);
-  });
-
-  test('empty list is a home narrow', () => {
-    expect(isHomeNarrow([])).toBe(true);
+  test('is a home narrow', () => {
+    expect(isHomeNarrow(HOME_NARROW)).toBe(true);
   });
 });
 
 describe('pmNarrowFromEmail', () => {
-  test('produces an one item list, pm-with operator and single email', () => {
-    expect(pmNarrowFromEmail(eg.otherUser.email)).toEqual([
-      {
-        operator: 'pm-with',
-        operand: eg.otherUser.email,
-      },
-    ]);
+  test('produces a 1:1 narrow', () => {
+    const narrow = pmNarrowFromEmail(eg.otherUser.email);
+    expect(is1to1PmNarrow(narrow)).toBeTrue();
+    expect(emailOfPm1to1Narrow(narrow)).toEqual(eg.otherUser.email);
   });
 
   test('if operator is "pm-with" and only one email, then it is a private narrow', () => {
     expect(is1to1PmNarrow(HOME_NARROW)).toBe(false);
     expect(is1to1PmNarrow(pmNarrowFromEmail(eg.otherUser.email))).toBe(true);
-    expect(
-      is1to1PmNarrow([
-        {
-          operator: 'pm-with',
-          operand: eg.otherUser.email,
-        },
-      ]),
-    ).toBe(true);
   });
 });
 
@@ -70,22 +58,6 @@ describe('isPmNarrow', () => {
     expect(isPmNarrow(HOME_NARROW)).toBe(false);
     expect(isPmNarrow(pmNarrowFromEmail(eg.otherUser.email))).toBe(true);
     expect(isPmNarrow(pmNarrowFromUsersUnsafe([eg.otherUser, eg.thirdUser]))).toBe(true);
-    expect(
-      isPmNarrow([
-        {
-          operator: 'pm-with',
-          operand: eg.otherUser.email,
-        },
-      ]),
-    ).toBe(true);
-    expect(
-      isPmNarrow([
-        {
-          operator: 'pm-with',
-          operand: [eg.otherUser.email, eg.thirdUser.email].join(','),
-        },
-      ]),
-    ).toBe(true);
   });
 });
 
@@ -104,80 +76,44 @@ describe('isStreamOrTopicNarrow', () => {
 });
 
 describe('specialNarrow', () => {
-  test('produces a narrow with "is" operator', () => {
-    expect(STARRED_NARROW).toEqual([
-      {
-        operator: 'is',
-        operand: 'starred',
-      },
-    ]);
-  });
-
   test('only narrowing with the "is" operator is special narrow', () => {
     expect(isSpecialNarrow(undefined)).toBe(false);
     expect(isSpecialNarrow(HOME_NARROW)).toBe(false);
     expect(isSpecialNarrow(streamNarrow('some stream'))).toBe(false);
     expect(isSpecialNarrow(STARRED_NARROW)).toBe(true);
-    expect(isSpecialNarrow([{ operator: 'stream', operand: 'some stream' }])).toBe(false);
-    expect(isSpecialNarrow([{ operator: 'is', operand: 'starred' }])).toBe(true);
   });
 });
 
 describe('streamNarrow', () => {
   test('narrows to messages from a specific stream', () => {
-    expect(streamNarrow('some stream')).toEqual([
-      {
-        operator: 'stream',
-        operand: 'some stream',
-      },
-    ]);
+    const narrow = streamNarrow('some stream');
+    expect(isStreamNarrow(narrow)).toBeTrue();
+    expect(streamNameOfNarrow(narrow)).toEqual('some stream');
   });
 
   test('only narrow with operator of "stream" is a stream narrow', () => {
     expect(isStreamNarrow(undefined)).toBe(false);
     expect(isStreamNarrow(HOME_NARROW)).toBe(false);
     expect(isStreamNarrow(streamNarrow('some stream'))).toBe(true);
-    expect(isStreamNarrow([{ operator: 'stream', operand: 'some stream' }])).toBe(true);
   });
 });
 
 describe('topicNarrow', () => {
   test('narrows to a specific topic within a specified stream', () => {
-    expect(topicNarrow('some stream', 'some topic')).toEqual([
-      { operator: 'stream', operand: 'some stream' },
-      { operator: 'topic', operand: 'some topic' },
-    ]);
+    const narrow = topicNarrow('some stream', 'some topic');
+    expect(isTopicNarrow(narrow)).toBeTrue();
+    expect(streamNameOfNarrow(narrow)).toEqual('some stream');
+    expect(topicOfNarrow(narrow)).toEqual('some topic');
   });
 
   test('only narrow with two items, one for stream, one for topic is a topic narrow', () => {
     expect(isTopicNarrow(undefined)).toBe(false);
     expect(isTopicNarrow(HOME_NARROW)).toBe(false);
     expect(isTopicNarrow(topicNarrow('some stream', 'some topic'))).toBe(true);
-    expect(
-      isTopicNarrow([
-        {
-          operator: 'stream',
-          operand: 'some stream',
-        },
-        {
-          operator: 'topic',
-          operand: 'some topic',
-        },
-      ]),
-    ).toBe(true);
   });
 });
 
 describe('SEARCH_NARROW', () => {
-  test('produces a narrow for a search query', () => {
-    expect(SEARCH_NARROW('some query')).toEqual([
-      {
-        operator: 'search',
-        operand: 'some query',
-      },
-    ]);
-  });
-
   test('narrow with "search" operand is a search narrow', () => {
     expect(isSearchNarrow(undefined)).toBe(false);
     expect(isSearchNarrow(HOME_NARROW)).toBe(false);
diff --git a/src/utils/message.js b/src/utils/message.js
index ba0bce5ef65..a5e3be9cead 100644
--- a/src/utils/message.js
+++ b/src/utils/message.js
@@ -1,6 +1,6 @@
 /* @flow strict-local */
 import type { Narrow, Message, MuteState, Outbox, Subscription } from '../types';
-import { isTopicNarrow } from './narrow';
+import { isHomeNarrow, isTopicNarrow } from './narrow';
 import { streamNameOfStreamMessage } from './recipient';
 
 export const isTopicMuted = (stream: string, topic: string, mute: MuteState = []): boolean =>
@@ -22,7 +22,7 @@ export const shouldBeMuted = (
 
   const streamName = streamNameOfStreamMessage(message);
 
-  if (narrow.length === 0) {
+  if (isHomeNarrow(narrow)) {
     const sub = subscriptions.find(x => x.name === streamName);
     if (!sub || !sub.in_home_view) {
       return true;
diff --git a/src/utils/narrow.js b/src/utils/narrow.js
index a40e1f5a019..5ffed03fe2b 100644
--- a/src/utils/narrow.js
+++ b/src/utils/narrow.js
@@ -1,7 +1,8 @@
 /* @flow strict-local */
+import invariant from 'invariant';
 import isEqual from 'lodash.isequal';
 
-import type { Narrow, Message, Outbox, User, UserOrBot } from '../types';
+import type { ApiNarrow, Message, Outbox, User, UserOrBot } from '../types';
 import {
   normalizeRecipientsSansMe,
   pmKeyRecipientsFromMessage,
@@ -13,6 +14,25 @@ import {
 
 /* eslint-disable no-use-before-define */
 
+/**
+ * A narrow.
+ *
+ * A narrow is the navigational property that defines what to show in the
+ * message list UI: it's a subset of messages which might be a conversation,
+ * a whole stream, a search, or a few other varieties.
+ *
+ * Much of the data we fetch is to support the message list UI, and so we
+ * keep much of it in data structures indexed by narrow.
+ *
+ * See also:
+ *  * `keyFromNarrow`, for converting into a string key good for indexing
+ *    into a data structure;
+ *  * `caseNarrow` and its relatives, for pattern-matching or destructuring;
+ *  * `ApiNarrow` for the form we put a narrow in when talking to the
+ *    server, and `apiNarrowOfNarrow` for converting to it.
+ */
+export opaque type Narrow = ApiNarrow;
+
 export const isSameNarrow = (narrow1: Narrow, narrow2: Narrow): boolean =>
   Array.isArray(narrow1) && Array.isArray(narrow2) && isEqual(narrow1, narrow2);
 
@@ -365,6 +385,54 @@ export const emailsOfGroupPmNarrow = (narrow: Narrow): string[] =>
     },
   });
 
+/**
+ * The "other" user's email for a 1:1 PM narrow; else error.
+ *
+ * With "other" in scare-quotes because for the self-PM narrow, this is the
+ * self user.
+ *
+ * Any caller of this probably should be getting a whole user object instead
+ * of a Narrow in the first place.  And then that wrinkle about the self-PM
+ * narrow vs. other 1:1 narrows is a UI decision that can get made
+ * explicitly at the appropriate spot.
+ */
+export const emailOfPm1to1Narrow = (narrow: Narrow): string =>
+  caseNarrowPartial(narrow, {
+    pm: emails => {
+      invariant(emails.length === 1, 'emailOfPm1to1Narrow: got group-PM narrow');
+      return emails[0];
+    },
+  });
+
+/**
+ * The "PM key recipients" emails for a PM narrow; else error.
+ *
+ * This is the same list of users that can appear in a `PmKeyRecipients` or
+ * `PmKeyUsers`, but contains only their emails.
+ */
+export const emailsOfPmNarrow = (narrow: Narrow): string[] =>
+  caseNarrowPartial(narrow, { pm: emails => emails });
+
+/**
+ * The stream name for a stream or topic narrow; else error.
+ *
+ * Most callers of this should probably be getting passed a stream name
+ * instead of a Narrow in the first place; or if they do handle other kinds
+ * of narrows, should be using `caseNarrow`.
+ */
+export const streamNameOfNarrow = (narrow: Narrow): string =>
+  caseNarrowPartial(narrow, { stream: name => name, topic: streamName => streamName });
+
+/**
+ * The topic for a topic narrow; else error.
+ *
+ * Most callers of this should probably be getting passed a topic (and a
+ * stream name) instead of a Narrow in the first place; or if they do handle
+ * other kinds of narrows, should be using `caseNarrow`.
+ */
+export const topicOfNarrow = (narrow: Narrow): string =>
+  caseNarrowPartial(narrow, { topic: (streamName, topic) => topic });
+
 export const isPmNarrow = (narrow?: Narrow): boolean =>
   !!narrow && caseNarrowDefault(narrow, { pm: () => true }, () => false);
 
@@ -391,6 +459,11 @@ export const isStreamOrTopicNarrow = (narrow?: Narrow): boolean =>
 export const isSearchNarrow = (narrow?: Narrow): boolean =>
   !!narrow && caseNarrowDefault(narrow, { search: () => true }, () => false);
 
+/**
+ * Convert the narrow into the form used in the Zulip API at get-messages.
+ */
+export const apiNarrowOfNarrow = (narrow: Narrow): ApiNarrow => narrow;
+
 /**
  * True just if the given message is part of the given narrow.
  *