Minecraft wiki entry for Chalkboard, associated text: 1×1 slates placed on the top of a block stand on a short post, facing in the direction of the player who placed it, in any of 16 different directions. 2×1 posters and 2×3 boards can be placed in any of 4 different directions. Chalkboards placed on the side of a block simply float there, even if the block doesn't make contact with the chalkboards.