For PCs, bays come in two basic sizes: 3.5-inch and 5.25-inch, which represents the bay's height. In addition, bays are described as either internal or exposed. An internal bay cannot be used for removable media, such as floppy drives. Some manufacturers use the terms hidden and accessible in place of internal and exposed.

Do not confuse bays with slots, which are openings in the computer where expansion boards can be installed.

PC system case reference guide
A complete look at the PC system case, including its major components, different sizes and styles, form factors, switches, LEDs and drive bays.