While many form factors are prevalent in pre-built systems, ATX, BTX and smaller derivatives dominate the build-your-own market. Of those, ATX is the most common (Advanced Technology Extended), as the newer BTX form factor (Balanced Technology Extended) still struggles to win the hearts of small-scale builders.

Reduced-size variations of ATX and BTX form factors limit room for expansion and increase reliance on integrated or external devices (but they allow compact system dimensions). Even as advancements in integrated audio and network controllers have improved these to the point that most enthusiasts require nothing more, improvements in integrated graphics have not been enough to scale them beyond their traditional roles in office-type systems. While office systems may serve a fixed role throughout their useful lives, it's a good idea to have at least some room for expansion cards when building systems for personal use.

ATX Through Mini-ITX

ATX was designed to address three major shortcomings of the earlier AT form factor, and it offers a few minor improvements as well. First, a designated portion of the board for the CPU socket keeps it out of the way of long cards, where AT boards had the CPU mounted behind or in place of card slots. Second, the inclusion of a port panel on the board itself negates the need for slot brackets to break out such common items as USB, serial, parallel or audio ports. Third, a cooling path from the lower front to the upper rear of the case vents hot air through the power supply and/or an exhaust fan. All three major improvements are centered on splitting the board between the slot area and CPU area.

Most significant of minor improvements was the addition of power-on through the motherboard. This allowed the system to turn itself off at shutdown and made possible such features as wake-on-ring (using a modem), wake-on-LAN (using a network adapter), timed power up/power down and keyboard power-on hot-buttons.

ATX derivatives are based on the same CPU section, so that smaller boards are able to fit into larger cases if desired. ATX standards include Micro ATX and Flex ATX: Most Shuttle style PC cubes (often called SFF for Shuttle Form Factor or Small Form Factor) use a 2-slot variation of the Flex ATX form factor reduced to approximately eight inches, and VIA further shortened its Mini ITX form factor to 6.75 inches by reducing the maximum slot count to one. ATX size specifications are based on fractional inches.

The image compares the maximum size and maximum number of slots allowed on various ATX-based form factors, with dashed lines indicating how the mounting holes in smaller boards still align with those of larger cases.