Motherboards are available in a variety of form factors, which usually correspond to a variety of computer case sizes. The following is a summary of some of the more popular PC motherboard sizes available:
* PC/XT - the original open motherboard standard created by IBM for the first home computer, the IBM-PC. It created a large number of 'clone' motherboards due to its open standard and therefore became the de facto standard.
* AT form factor (Advanced Technology) - the first form factor to gain wide acceptance, successor to PC/XT. Also known as Full AT, it was popular during the 386 era. Obsolete - superseded by ATX.
* Baby AT - IBM's successor to the AT motherboard, it was functionally equivalent to the AT but gained popularity due to its significantly smaller physical size, usually comes without AGP port.
* ATX - the 'evolution' of the Baby AT form factor, it is now the most popular form factor available today.
* Mini-ATX - essentially the same as the ATX layout, but again, with a smaller 'footprint'.
* microATX - again, a miniaturization of the ATX layout. It is commonly used in the larger of the 'cube'-style cases such as the Antec ARIA.
* FlexATX - a subset of microATX allowing more flexible motherboard design, component positioning and shape.
* LPX - based on a design by Western Digital, it allows for smaller cases based on the ATX motherboard by arranging the expansion cards in a riser (an expansion card in itself, attaching to the side of the motherboard - image). This design allows the cards to rest parallel to the motherboard as opposed to perpendicular to it. The LPX motherboard is generally only used by large OEM manufacturers.
* Mini LPX - a smaller subset of the LPX specification.
* NLX - a 'low-profile' motherboard, again incorporating a 'riser', designed in order to 'keep up with market trends'. NLX never gained much popularity.
* BTX (Balanced Technology Extended) - a newer standard proposed by Intel as an eventual successor to ATX.
* microBTX and picoBTX - smaller subsets of the BTX standard.
* Mini-ITX - VIA's highly-integrated small form factor motherboard, designed for uses including thin clients and Set-top box.
* WTX (Workstation Technology Extended) - a large motherboard (more so than ATX) designed for use with high-power workstations (usually featuring multiple processors or hard drives.
While most desktop PCs use one of these motherboard form factors, notebook computers generally use highly integrated, customized and miniaturized motherboards designed by the manufacturers. This is one of the reasons that notebook computers are difficult to upgrade and expensive to repair - often the failure of one integrated component requires the replacement of the entire motherboard, which is also more expensive than a regular motherboard due to the large number of integrated components in it.
