The 5 1/4" drives come in two distinct flavors: full height taking a full slot in your PC's bay or half height which takes up a half slot in your PC's bay. The full height drives are older technology, and generally not used on the newer PCs. In fact, all PCs today only come with half-height bays.

The 3 1/2" drives come only in the half-height flavor. Some of these drives take up less than a half height bay using a special 3 1/2" drive slot.