ATX 【ATX form factor】
full spelling ： Advanced Technology Extended
In 1995 Intel introduced ATX form factor. ATX overtook AT completely as the default form factor for new systems. It was the first big change in computer case and motherboard design in many years. The ATX form factor specified changes to the motherboard, along with the case and power supply. ATX form factor included a single 20-pin connector for the power supply.
The standard ATX is 305 mm x 244 mm in size. The expansion slots were put onto separate riser cards that plugged into the motherboard. This has reduced the overall size of the computer and its case.