Cases can come in many different sizes (known as "form factors"). The size and shape of a computer case are usually determined by the form factor of the motherboard since it is the largest component of most computers. Cases are also referred to by their size (such as ATX case, Mini ITX case, etc.) 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.