diff --git a/doc/formats/sets.md b/doc/formats/sets.md
index b7d7f8d..46aeb67 100644
--- a/doc/formats/sets.md
+++ b/doc/formats/sets.md
@@ -75,6 +75,10 @@ height indicator (presuambly), are shown when referenced as `pillar_2`:
 
 | ![pillar.obj as pillar](img/pillar_6_sprites.png) | |[pillar.obj as pillar_2](img/pillar_2_sprites_meta.png) |
 
+When referenced as `pillar_3`, the middle section is rendered twice, rather than
+once. Only 2 sprites are shown, and the blue "height" indicator increases from
+4 to 5.
+
 ## Remaining questions
 
 Do positions in the palette have special meaning? e.g. is a particular range