(from DM guide:)
Layer default value:
1 (AREA_LAYER)
2 (TURF_LAYER)
3 (OBJ_LAYER)
4 (MOB_LAYER)

1 (A)
2 (T)
3 (O)
4 (M)

Atom. Dan/Tom, you clever dog, you.
(Credits to Charlesg154 for noticing)