ATX is a specific standard format, used on an industrial level, that defines dimensions and characteristics of PC motherboards. The existence of a shared and recognized standard allows users and PC vendors to buy motherboards coming from different manufacturers without incurring in hardware or dimension incompatibilities.

The ATX format was born as an evolution of the previous original AT format by rotating by 90° the processor and expansion slots layout to leave more freedom while installing additional peripheral.

The other main formats are Micro-ATX and Mini-ATX.