Full tower cases are typically 56 cm (22 in) or more in height and intended to stand on the floor. They have anywhere from six to ten externally accessible drive bays. The ratio of external to internal bays is shifting, however, as computing technology moves from floppy disks and CD-ROMs to large capacity hard drives, USB flash drives, and network-based solutions. The full tower case was developed to house file servers which would typically be tasked with serving data from expensive CD-ROM databases which held more data than the hard drives commonly available. Hence many full tower cases include locking doors and other physical security features to prevent theft of the discs. This is a high-end case intended for desktop systems and doesn't include security features.
Mid-tower cases are smaller, about 46 cm (18 in) high with two to four external bays.
A mini-tower case will typically have only one or two external bays.[2]
Recently the marketing term midi-tower has come into use, seemingly referring to (based on anecdotal evidence) cases smaller than mid-tower but larger than mini-tower, typically with two to three external bays. Outside of the United States the term is often used interchangeably with mid-tower.
The computer case is sometimes erroneously referred to as the "CPU" or "hard drive