A bunkie (also spelled "bunky") board is a thin piece of furniture that is placed below the mattress of a bunk bed. It supports the mattress so that it is sturdy and the sleeper is comfortable.
Bunkie boards are used in place of box springs, when the added height that box springs create is not needed. They are most often used for bunk beds because head clearance is important. Bunkies also help to extend the life of a mattress because they prevent sagging underneath the bed frame. They are typically one to two inches thick.
The tops of bunkie boards are covered in a thin layer of fabric to help keep the mattress soft and provide traction between the mattress and the bed frame. They have become more popular with the rise in popularity of the platform bed, which also does not need the extra height provided by a box spring. Bunkie boards offer more support than slats and are often made of thin wood so that they are inexpensive and keep the mattress low. They can also be referred to as mattress supports. A bunkie board is simply placed on top of the bed's slats and then the mattress is placed on top of it.