Dirt Wall

Can be placed like any other backing wall, however it does not stop enemies from spawning, so would not be suitable for a dwelling. As of Update 1.06, NPCs can live in houses made with a placed dirt wall background, but not a natural one. Note though, that placed dirt wall is not compatiable with world generated dirt walls, they will not merge with each other, the game treats them as different kinds of background, this may be a bug introduced in 1.06.