ATX is an industry-wide specification for a desktop computer's motherboard. ATX improves the motherboard design by taking the small AT motherboard (sometimes known as the "Baby AT" or BAT) that was an earlier industry standard and rotating by 90 degrees the layout of the microprocessor and expansion slots. This allows space for more full-length add-in cards. A double-height aperture is specified for the rear of the chassis, allowing more possible I/O arrangements for a variety of devices such as TV input and output, LAN connection, and so forth. The new layout is also intended to be less costly to manufacture. Fewer cables are needed. The power supply has a side-mounted fan, allowing direct cooling of the processor and cards, making a secondary fan unnecessary.
Almost all major computer manufacturers, including IBM, Compaq, and Apple build desktops with ATX motherboards. IBM is using ATX in both Intel and PowerPC platforms.