A computer chassis (pronounced cha-see) is the frame that houses the internal components of a computer, such as the motherboard, power supply and hard drives. Chassis are typically based on a standard form factor that defines the physical dimensions of the chassis and the components it houses in order to ensure compatibility and interchangeability. ATX has been the most common form factor used in desktop computers for many years and has three variations: standard ATX, micro-ATX and mini-ATX