The Motherboard Form Factor ATX
Starting in 1995, Intel designed and introduced the motherboard form factor ATX. This motherboard was smaller than the IBM form factor, until then the standard desktop motherboard, and was about 12 x 12 inches in size. The ATX form factor defines several key mechanical dimensions like the I/O panel, the mounting point, connector and power interfaces between a motherboard, computer case, and the power supply. The newer form factor introduced new circuitry technology making the motherboard cooler, and had new design features which have stood up for 15 years.
In size the ATX was 12 x 9.6 inches. But it was the introduction of the riser that did much to change how components could be added to the motherboard.The Audio Modem Riser is an Intel specification for the design of motherboards. This spec allows manufacturers to create motherboards without analog I/O functions.
Now these functions appear on a separate card. This allowed newer technologies to be added to a smaller motherboard.
This image shows the basic design configuration and the circuitry. These were new features not found on the previous AT form factor.
One more thing; the ATX also spawned additional motherboard designs that fit the mantra: smaller is better. These were the Micro ATX, the Mini ATX, and the Flex ATX. These are all discussed below.