A computer case (also known as a computer chassis, cabinet, box, tower, enclosure, housing, system unit or simply case) is the enclosure that contains most of the components of a computer (usually excluding the display, keyboard and mouse). A computer case is sometimes incorrectly referred to metonymously as a CPU or hard drive referring to components housed within the case. CPU was a more common term in the earlier days of home computers, when peripherals other than the motherboard were usually housed in their own separate cases.
Cases are usually constructed from steel (often SECC—Steel, Electrically Chromate Coated) or aluminium. Plastic is sometimes used, and other materials such as wood and even Lego blocks have appeared in home-built cases. Cases can come in many different sizes (known as form factors). The size and shape of a computer case is usually determined by the form factor of the motherboard, since it is the largest component of most computers. Consequently, personal computer form factors typically specify only the internal dimensions and layout of the case. Form factors for rack-mounted and blade servers may include precise external dimensions as well, since these cases must themselves fit in specific enclosures.