diff --git a/README.md b/README.md
index e3315f1..3566415 100644
--- a/README.md
+++ b/README.md
@@ -43,6 +43,8 @@ keypresses:
 
  - Reload the config file: Press the "R" key.
 
+ - Switch between rendering modes: Press the "M" key.
+
  - Quit the program: Close the window, or press the escape key.
 
 Configuring the L-System
diff --git a/l_system_3d.c b/l_system_3d.c
index 0bbddbd..af6bea7 100644
--- a/l_system_3d.c
+++ b/l_system_3d.c
@@ -16,6 +16,7 @@
 #define DEFAULT_WINDOW_WIDTH (800)
 #define DEFAULT_WINDOW_HEIGHT (600)
 #define DEFAULT_FPS (60.0)
+#define DEFAULT_GEOMETRY_THICKNESS (0.5)
 
 static ApplicationState* AllocateApplicationState(void) {
   ApplicationState *to_return = NULL;
@@ -26,6 +27,7 @@ static ApplicationState* AllocateApplicationState(void) {
   to_return->aspect_ratio = ((float) to_return->window_width) /
     ((float) to_return->window_height);
   to_return->frame_duration = 1.0 / DEFAULT_FPS;
+  to_return->shared_uniforms.geometry_thickness = DEFAULT_GEOMETRY_THICKNESS;
   return to_return;
 }
 
@@ -98,6 +100,7 @@ static int GenerateVertices(ApplicationState *s) {
   int result;
   uint32_t char_index, inst_index;
   uint8_t c;
+  float size_scale;
   ResetTurtle3D(t);
   for (char_index = 0; char_index < s->l_system_length; char_index++) {
     c = s->l_system_string[char_index];
@@ -117,10 +120,11 @@ static int GenerateVertices(ApplicationState *s) {
     return 0;
   }
   if (!SetTransformInfo(s->turtle, s->mesh->model, s->mesh->normal,
-    s->mesh->location_offset)) {
+    s->mesh->location_offset, &size_scale)) {
     printf("Failed getting transform matrices.\n");
     return 0;
   }
+  s->shared_uniforms.size_scale = size_scale;
   return 1;
 }
 
@@ -233,6 +237,10 @@ static int ProcessInputs(ApplicationState *s) {
     glfwSetWindowShouldClose(s->window, 1);
     return 1;
   }
+  // TODO: Replace this key-press logic with a single function that checks
+  // the state of key_pressed_tmp, setting or unsetting it if necessary and
+  // returning true on a "new" press.
+
   // s->key_pressed_tmp is used to prevent counting one press multiple times,
   // and to prevent up and down from being pressed together.
   pressed = glfwGetKey(s->window, GLFW_KEY_UP) == GLFW_PRESS;
@@ -265,6 +273,18 @@ static int ProcessInputs(ApplicationState *s) {
     // R pressed -> R released
     s->key_pressed_tmp = 0;
   }
+  pressed = glfwGetKey(s->window, GLFW_KEY_M) == GLFW_PRESS;
+  if (!s->key_pressed_tmp && pressed) {
+    // Nothing pressed -> M pressed
+    s->key_pressed_tmp = GLFW_KEY_M;
+    if (!SwitchRenderingModes(s->mesh)){
+      printf("Failed switching rendering modes.\n");
+      return 0;
+    }
+  } else if ((s->key_pressed_tmp == GLFW_KEY_M) && !pressed) {
+    // M pressed -> M released
+    s->key_pressed_tmp = 0;
+  }
   return 1;
 }
 
@@ -303,8 +323,9 @@ static int WaitNextFrame(ApplicationState *s) {
 
 static int RunMainLoop(ApplicationState *s) {
   glEnable(GL_DEPTH_TEST);
-  glEnable(GL_CULL_FACE);
-  glCullFace(GL_BACK);
+  // TODO: Re-enable face culling after getting the geometry shader working.
+  //glEnable(GL_CULL_FACE);
+  //glCullFace(GL_BACK);
   glClearColor(0, 0, 0, 1.0);
   while (!glfwWindowShouldClose(s->window)) {
     s->frame_start = glfwGetTime();
diff --git a/l_system_3d.h b/l_system_3d.h
index f714afe..868572d 100644
--- a/l_system_3d.h
+++ b/l_system_3d.h
@@ -12,6 +12,9 @@ typedef struct {
   mat4 projection;
   mat4 view;
   vec4 camera_position;
+  float size_scale;
+  float geometry_thickness;
+  float pad[2];
 } SharedUniforms;
 
 // Maintains global data about the running program.
diff --git a/l_system_mesh.c b/l_system_mesh.c
index 2e9d0a8..0414395 100644
--- a/l_system_mesh.c
+++ b/l_system_mesh.c
@@ -79,11 +79,13 @@ static GLuint LoadShader(const char *path, GLenum shader_type) {
 }
 
 // Links and returns the shader program from the given source file names.
+// The geometry_src_file can be NULL if a geometry shader isn't used. A
+// vertex and fragment shader are always required.
 static GLuint CreateShaderProgram(const char *vertex_src_file,
-    const char *fragment_src_file) {
+    const char *geometry_src_file, const char *fragment_src_file) {
   GLchar link_log[512];
   GLint link_result = 0;
-  GLuint vertex_shader, fragment_shader, to_return;
+  GLuint vertex_shader, geometry_shader, fragment_shader, to_return;
 
   vertex_shader = LoadShader(vertex_src_file, GL_VERTEX_SHADER);
   if (!vertex_shader) {
@@ -96,11 +98,28 @@ static GLuint CreateShaderProgram(const char *vertex_src_file,
     glDeleteShader(vertex_shader);
     return 0;
   }
+
+  // Only load the geometry shader if requested.
+  geometry_shader = 0;
+  if (geometry_src_file != NULL) {
+    geometry_shader = LoadShader(geometry_src_file, GL_GEOMETRY_SHADER);
+    if (!geometry_shader) {
+      printf("Couldn't load geometry shader.\n");
+      glDeleteShader(vertex_shader);
+      glDeleteShader(fragment_shader);
+      return 0;
+    }
+  }
+
   to_return = glCreateProgram();
   glAttachShader(to_return, vertex_shader);
+  if (geometry_shader) {
+    glAttachShader(to_return, geometry_shader);
+  }
   glAttachShader(to_return, fragment_shader);
   glLinkProgram(to_return);
   glDeleteShader(vertex_shader);
+  glDeleteShader(geometry_shader);
   glDeleteShader(fragment_shader);
   glGetProgramiv(to_return, GL_LINK_STATUS, &link_result);
   memset(link_log, 0, sizeof(link_log));
@@ -131,11 +150,11 @@ static int UniformIndex(GLuint program, const char *name, GLint *index) {
 
 // Loads the shaders for the model, and looks up uniform indices. Returns 0 on
 // error.
-static int SetupShaderProgram(LSystemMesh *m) {
+static int SetupShaderProgram(LSystemMesh *m, const char *vertex_src,
+    const char *geometry_src, const char *fragment_src) {
   GLuint p;
   GLuint block_index;
-  p = CreateShaderProgram("./mesh_vertex_shader.vert",
-    "./mesh_fragment_shader.frag");
+  p = CreateShaderProgram(vertex_src, geometry_src, fragment_src);
   if (!p) return 0;
   m->shader_program = p;
   if (!UniformIndex(p, "model", &(m->model_uniform_index))) return 0;
@@ -154,6 +173,19 @@ static int SetupShaderProgram(LSystemMesh *m) {
   return CheckGLErrors();
 }
 
+int SwitchRenderingModes(LSystemMesh *m) {
+  glDeleteProgram(m->shader_program);
+  m->shader_program = 0;
+  if (m->using_geometry_shader) {
+    m->using_geometry_shader = 0;
+    return SetupShaderProgram(m, "simple_shader.vert", NULL,
+      "simple_shader.frag");
+  }
+  m->using_geometry_shader = 1;
+  return SetupShaderProgram(m, "pipes_shader.vert", "pipes_shader.geom",
+    "pipes_shader.frag");
+}
+
 LSystemMesh* CreateLSystemMesh(void) {
   LSystemMesh *m = NULL;
 
@@ -186,7 +218,8 @@ LSystemMesh* CreateLSystemMesh(void) {
     DestroyLSystemMesh(m);
     return NULL;
   }
-  if (!SetupShaderProgram(m)) {
+  if (!SetupShaderProgram(m, "simple_shader.vert", NULL,
+    "simple_shader.frag")) {
     DestroyLSystemMesh(m);
     return NULL;
   }
diff --git a/l_system_mesh.h b/l_system_mesh.h
index 0388605..f84662b 100644
--- a/l_system_mesh.h
+++ b/l_system_mesh.h
@@ -30,6 +30,9 @@ typedef struct {
   uint32_t vertex_count;
   // OpenGL stuff needed for drawing this mesh.
   GLuint shader_program;
+  // If nonzero, the shader program is currently the more complex geometry
+  // version. If zero, we're just rendering the wireframe.
+  int using_geometry_shader;
   GLuint vao;
   GLuint vbo;
   // The model and normal matrices used when drawing this mesh.
@@ -62,4 +65,8 @@ int SetMeshVertices(LSystemMesh *m, MeshVertex *vertices, uint32_t count);
 // Draws the mesh. Returns 0 on error, including any GL errors if they occur.
 int DrawMesh(LSystemMesh *m);
 
+// Cycles between rendering modes (i.e. shader programs) that may be used to
+// render the given mesh. Returns 0 on error.
+int SwitchRenderingModes(LSystemMesh *m);
+
 #endif  // L_SYSTEM_MESH_H
diff --git a/pipes_shader.frag b/pipes_shader.frag
new file mode 100644
index 0000000..196b0c1
--- /dev/null
+++ b/pipes_shader.frag
@@ -0,0 +1,18 @@
+#version 330 core
+
+in GS_OUT {
+  vec3 frag_position;
+  vec3 forward;
+  vec3 up;
+  vec3 right;
+  vec4 color;
+} fs_in;
+
+
+out vec4 frag_color;
+
+//INCLUDE_SHARED_UNIFORMS
+
+void main() {
+  frag_color = fs_in.color;
+}
diff --git a/pipes_shader.geom b/pipes_shader.geom
new file mode 100644
index 0000000..48707ff
--- /dev/null
+++ b/pipes_shader.geom
@@ -0,0 +1,57 @@
+#version 330 core
+layout (lines) in;
+layout (triangle_strip, max_vertices = 4) out;
+
+in VS_OUT {
+  vec3 position;
+  vec3 forward;
+  vec3 up;
+  vec3 right;
+  vec4 color;
+} gs_in[];
+
+out GS_OUT {
+  vec3 frag_position;
+  vec3 forward;
+  vec3 up;
+  vec3 right;
+  vec4 color;
+} gs_out;
+
+//INCLUDE_SHARED_UNIFORMS
+
+void main() {
+  vec3 right = gs_in[0].right;
+  vec3 forward = gs_in[0].forward;
+  vec3 bottom = gs_in[0].position;
+  vec3 top = gs_in[0].position;
+  // NOTE: verify this is in the correct order.
+  mat4 projView = shared_uniforms.projection * shared_uniforms.view;
+  // The normal vectors don't change.
+  gs_out.forward = forward;
+  gs_out.up = gs_in[0].up;
+  gs_out.right = right;
+  //gs_out.color = gs_in[1].color;
+  gs_out.color = vec4(0.5, 0.3, 1.0, 1.0);
+
+  float half_rect_width = 0.5 * shared_uniforms.geometry_thickness *
+    shared_uniforms.size_scale;
+
+  // Bottom left
+  gs_out.frag_position = bottom - (right * half_rect_width);
+  gl_Position = projView * vec4(gs_out.frag_position, 1.0);
+  EmitVertex();
+  // Bottom right
+  gs_out.frag_position = bottom + (right * half_rect_width);
+  gl_Position = projView * vec4(gs_out.frag_position, 1.0);
+  EmitVertex();
+  // Top right
+  gs_out.frag_position = top + (right * half_rect_width);
+  gl_Position = projView * vec4(gs_out.frag_position, 1.0);
+  EmitVertex();
+  // Top left
+  gs_out.frag_position = top - (right * half_rect_width);
+  gl_Position = projView * vec4(gs_out.frag_position, 1.0);
+  EmitVertex();
+  EndPrimitive();
+}
diff --git a/pipes_shader.vert b/pipes_shader.vert
new file mode 100644
index 0000000..0a46638
--- /dev/null
+++ b/pipes_shader.vert
@@ -0,0 +1,29 @@
+#version 330 core
+layout (location = 0) in vec3 position_in;
+layout (location = 1) in vec3 forward_in;
+layout (location = 2) in vec3 up_in;
+layout (location = 3) in vec4 color_in;
+
+// Added to the location of each vertex to center the overall mesh on 0,0,0.
+uniform vec3 location_offset;
+
+uniform mat4 model;
+uniform mat3 normal;
+
+out VS_OUT {
+  vec3 position;
+  vec3 forward;
+  vec3 up;
+  vec3 right;
+  vec4 color;
+} vs_out;
+
+//INCLUDE_SHARED_UNIFORMS
+
+void main() {
+  vs_out.position = (model * vec4(position_in + location_offset, 1.0)).xyz;
+  vs_out.forward = normalize(normal * forward_in);
+  vs_out.up = normalize(normal * up_in);
+  vs_out.right = normalize(cross(vs_out.forward, vs_out.up));
+  vs_out.color = color_in;
+}
diff --git a/shared_uniforms.glsl b/shared_uniforms.glsl
index a7ccdfa..80c4d7c 100644
--- a/shared_uniforms.glsl
+++ b/shared_uniforms.glsl
@@ -2,5 +2,8 @@ layout(std140) uniform SharedUniforms {
   mat4 projection;
   mat4 view;
   vec4 camera_position;
+  float size_scale;
+  float geometry_thickness;
+  float pad[2];
 } shared_uniforms;
 
diff --git a/mesh_fragment_shader.frag b/simple_shader.frag
similarity index 85%
rename from mesh_fragment_shader.frag
rename to simple_shader.frag
index 6129ccc..c1514da 100644
--- a/mesh_fragment_shader.frag
+++ b/simple_shader.frag
@@ -2,6 +2,9 @@
 
 in VS_OUT {
   vec3 frag_position;
+  vec3 forward;
+  vec3 up;
+  vec3 right;
   vec4 color;
 } fs_in;
 
diff --git a/mesh_vertex_shader.vert b/simple_shader.vert
similarity index 58%
rename from mesh_vertex_shader.vert
rename to simple_shader.vert
index cce296d..4e106df 100644
--- a/mesh_vertex_shader.vert
+++ b/simple_shader.vert
@@ -12,6 +12,9 @@ uniform vec3 location_offset;
 
 out VS_OUT {
   vec3 frag_position;
+  vec3 forward;
+  vec3 up;
+  vec3 right;
   vec4 color;
 } vs_out;
 
@@ -22,12 +25,15 @@ void main() {
   gl_Position = shared_uniforms.projection * shared_uniforms.view * model *
     vec4(position_in + location_offset, 1.0);
   vs_out.color = color_in;
+  vs_out.forward = normal * forward_in;
+  vs_out.up = normal * up_in;
+  vs_out.right = normalize(cross(vs_out.forward, vs_out.up));
 
-  // Prevent optimizing out the normal uniform.
-  // TODO (eventually): Remove once we actually use the normal uniform.
-  if (normal[0].x < -1) {
-    vs_out.color[1] = 1;
+  // Used to prevent the normal matrix from being optimized out. (I don't want
+  // to have to ignore the error when looking for its uniform location.)
+  if (normal[0][0] == 0.0001) {
+    vs_out.color[0] *= 0.9999;
   }
 
-  vs_out.frag_position = vec3(model * vec4(position_in, 1.0));
+  vs_out.frag_position = vec3(model * vec4(position_in + location_offset, 1.0));
 }
diff --git a/turtle_3d.c b/turtle_3d.c
index c9b022f..596f039 100644
--- a/turtle_3d.c
+++ b/turtle_3d.c
@@ -128,8 +128,10 @@ static void ModelToNormalMatrix(mat4 model, mat3 normal) {
   glm_mat4_pick3(dst, normal);
 }
 
-int SetTransformInfo(Turtle3D *t, mat4 model, mat3 normal, vec3 loc_offset) {
+int SetTransformInfo(Turtle3D *t, mat4 model, mat3 normal, vec3 loc_offset,
+    float *size_scale) {
   float dx, dy, dz, max_axis;
+  *size_scale = 1.0;
   dx = t->max_bounds[0] - t->min_bounds[0];
   dy = t->max_bounds[1] - t->min_bounds[1];
   dz = t->max_bounds[2] - t->min_bounds[2];
@@ -145,7 +147,8 @@ int SetTransformInfo(Turtle3D *t, mat4 model, mat3 normal, vec3 loc_offset) {
   max_axis = Max3(dx, dy, dz);
   glm_mat4_identity(model);
   if (max_axis > 0) {
-    glm_scale_uni(model, MESH_CUBE_SIZE / max_axis);
+    *size_scale = MESH_CUBE_SIZE / max_axis;
+    glm_scale_uni(model, *size_scale);
   }
   ModelToNormalMatrix(model, normal);
   // TODO (eventually): If I get less stupid, combine the loc_offset
diff --git a/turtle_3d.h b/turtle_3d.h
index d9117c7..3b34fc6 100644
--- a/turtle_3d.h
+++ b/turtle_3d.h
@@ -78,8 +78,10 @@ void DestroyTurtle3D(Turtle3D *t);
 
 // Sets the model and normal matrices for the turtle's vertices, and the
 // location offset vector (to be added to each vertex location) that will
-// center the mesh on 0, 0, 0. Returns 0 on error.
-int SetTransformInfo(Turtle3D *t, mat4 model, mat3 normal, vec3 loc_offset);
+// center the mesh on 0, 0, 0. Returns 0 on error. Also returns the linear
+// scale by which the model matrix scales up or down.
+int SetTransformInfo(Turtle3D *t, mat4 model, mat3 normal, vec3 loc_offset,
+    float *size_scale);
 
 // The type used by all turtle-movement instructions. Some instructions may not
 // use the floating-point parameter. All instructions must return 0 on error