diff --git a/.claude/rules/unicode-graphemes.md b/.claude/rules/unicode-graphemes.md
new file mode 100644
index 0000000000..c1b7ec5b11
--- /dev/null
+++ b/.claude/rules/unicode-graphemes.md
@@ -0,0 +1,48 @@
+# Unicode and Grapheme Handling
+
+**Source:** [CONTRIBUTING.md - Unicode and Grapheme Handling](../../CONTRIBUTING.md#unicode-and-grapheme-handling)
+
+## Core Principle
+
+**Think in graphemes, not runes.** A grapheme cluster is what the user perceives as a single character, but it may consist of multiple `Rune` values (e.g., base character + combining marks, or ZWJ emoji sequences).
+
+## Width Measurement
+
+**Always use `string.GetColumns()`** to measure display width.
+
+```csharp
+// ✅ CORRECT — grapheme-aware, handles ZWJ emoji, combining marks, CJK
+int width = text.GetColumns ();
+
+// ❌ WRONG — sums individual rune widths, inflates multi-rune grapheme clusters
+int width = text.EnumerateRunes ().Sum (r => r.GetColumns ());
+
+// ❌ WRONG — counts chars, not terminal cells (wrong for CJK, emoji, surrogates)
+int width = text.Length;
+```
+
+**Why:** A ZWJ family emoji like 👨‍👩‍👦‍👦 occupies 2 terminal cells, but `EnumerateRunes().Sum(GetColumns)` returns 8 and `string.Length` returns 11. `string.GetColumns()` correctly returns 2 by iterating grapheme clusters and clamping each to max 2 columns.
+
+## Text Iteration and Rendering
+
+**Iterate by grapheme cluster** using `GraphemeHelper.GetGraphemes()` and render with `AddStr`.
+
+```csharp
+// ✅ CORRECT — preserves grapheme cluster integrity
+foreach (string grapheme in GraphemeHelper.GetGraphemes (text))
+{
+    AddStr (grapheme);
+}
+
+// ❌ WRONG — breaks combining marks, ZWJ sequences
+foreach (Rune rune in text.EnumerateRunes ())
+{
+    AddRune (rune);
+}
+```
+
+**Why:** Combining marks (e.g., `e` + U+0301 combining acute = `é`) must be sent together. `AddRune` sends them separately, preventing composition. `AddStr` with a complete grapheme string preserves the cluster.
+
+## When Rune Iteration Is Appropriate
+
+Rune-level iteration is correct when you need to inspect individual Unicode scalar values rather than render text — for example, counting zero-width runes for vertical text layout, or checking character properties. The key distinction: **rendering and measurement should use graphemes; analysis may use runes.**
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 0b1abf6c57..f7bfc635b6 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -169,6 +169,36 @@ Welcome! This guide provides everything you need to know to contribute effective
 
 **⚠️ CRITICAL - These conventions apply to ALL code - production code, test code, examples, documentation, and samples.**
 
+### Unicode and Grapheme Handling
+
+**Think in graphemes, not runes.** A grapheme cluster is what the user perceives as a single character, but it may consist of multiple `Rune` values (e.g., base character + combining marks, or ZWJ emoji sequences).
+
+- **Always use `string.GetColumns()`** to measure display width — never `EnumerateRunes().Sum(r => r.GetColumns())` (inflates multi-rune clusters) or `string.Length` (counts chars, not terminal cells)
+- **Iterate by grapheme** using `GraphemeHelper.GetGraphemes()` when rendering text — never iterate by `Rune` and call `AddRune` for each (breaks combining marks and ZWJ sequences)
+- **Render with `AddStr`** passing complete grapheme strings — `AddRune` with individual runes from a cluster will not compose correctly
+
+```csharp
+// ✅ CORRECT — grapheme-aware width measurement
+int width = text.GetColumns ();
+
+// ❌ WRONG — inflates width for ZWJ emoji (e.g., 👨‍👩‍👦‍👦 → 8 instead of 2)
+int width = text.EnumerateRunes ().Sum (r => r.GetColumns ());
+
+// ✅ CORRECT — grapheme-aware rendering
+foreach (string grapheme in GraphemeHelper.GetGraphemes (text))
+{
+    AddStr (grapheme);
+}
+
+// ❌ WRONG — breaks combining marks (é rendered as e + ́ separately)
+foreach (Rune rune in text.EnumerateRunes ())
+{
+    AddRune (rune);
+}
+```
+
+**Exception:** Rune-level iteration is appropriate when inspecting individual Unicode scalar values (e.g., counting zero-width runes for vertical text layout), not for rendering or measurement.
+
 ## Testing Requirements
 
 ### Code Coverage
@@ -324,5 +354,7 @@ The workflow will:
 - ❌ **Don't use `var` for anything but built-in simple types** (use explicit types)
 - ❌ **Don't use redundant type names with `new`** (**ALWAYS PREFER** target-typed `new ()`)
 - ❌ **Don't introduce new warnings** (fix warnings in files you modify; exception: `[Obsolete]` warnings)
+- ❌ **Don't use `EnumerateRunes().Sum(GetColumns)`** for display width — use `string.GetColumns()`
+- ❌ **Don't use `AddRune` in a rune loop** to render text — iterate by grapheme with `GraphemeHelper.GetGraphemes()` and use `AddStr`
 
 **Thank you for contributing to Terminal.Gui!** 🎉