Minecraft wiki entry for Walls_and_buttresses, associated text: Picture rail − a thin wooden rail at slightly above head height which provided a way to hang pictures on a plastered wall. You can't drive nails into plaster itself because that is highly likely to break it. Hence the wooden picture rail decided the only height at which you could hang paintings in a room with plastered walls. The picture rail often also gave the upper border for a room's wallpaper. Above the picture rail, wall and ceiling would usually be the same color.