diff --git a/gdd.md b/gdd.md
index 6638cae..fa2b769 100644
--- a/gdd.md
+++ b/gdd.md
@@ -8,6 +8,11 @@
 * Education
 * Supply chains
 
+## Clock
+
+Ticks can be applied manually or they can be applied over time.
+Add a tick counter as a UI element.
+
 ## Grid
 
 Add a 10x10 grid.
@@ -17,13 +22,6 @@ Tiles are highlighted a different colour when the player hovers over them with t
 
 Tiles in the grid contain floating point values.
 These values propogate to neighbouring cells a tick happens.
-Ticks can be applied manually or they can be applied over time.
-Add a tick counter as a UI element.
-
-* [ ] understand different options for representing the rate of diffusion
-
-Diffusing should use 'pull' operations; tiles should pull values from surrounding tiles.
-This is an attempt to think forwards to parallelising computation of diffusiong - ideally using the GPU.
 
 #### Diffusion Algorithm
 
@@ -46,6 +44,20 @@ private float TransferHeat(float t0, float alpha, float nx, float ny, float px,
 }
 ```
 
+Diffusing uses 'pull' operations; tiles pull values from surrounding tiles.
+This is an attempt to think forwards to parallelising computation of diffusiong - ideally using the GPU.
+
+## Overlays
+
+The current overlay can be cycled with a UI button or by pressing `tab`.
+Overlays display tile information by displaying tiles in different colours.
+Normally, tile colours are displayed depending on their type and health.
+This is the 'normal' view with no overlay.
+
+Other overlays include:
+
+* [Heat overlay](#heat-overlay)
+
 ## Build Mode
 
 Build mode can be entered by pressing the `b` key.
@@ -71,8 +83,12 @@ In build mode, the player can click on a wile tile to turn it into a developed t
 
 Developed tiles produce heat while wild tiles absorb it.
 Every tile has a floating-point heat value which diffuses to its neighbours.
-Add a heat map view which can be toggled or cycled to.
+
+### Heat overlay
+
+A heat map view can be toggled or cycled to.
 It is possible to cycle the view using a UI element.
+Tile colours are displayed as a value along a gradient from cold to hot tiles.
 
 ## Tile Health