Platforms - The Official Terraria Wiki

In the default or "top" position, it occupies the top half of a block space. Only in this position can furniture be placed on top of it. This includes a Bottle (or similar), which will count as a Placed Bottle for crafting potions.After one hammer blow, it will change to the slanted stairs form, facing right. It may also show "struts" beneath the slanted part depending on adjacent blocks, but this is purely visual.A second hammer blow will switch it to stairs facing left.A third hammer blow will move it to its "bottom" position, horizontal but lowered to the bottom half of the block. Only in this position, banners (and some other hanging items) can be attached beneath them. However, chandeliers or lanterns will not attach there.