ATX is an extension of the AT specification. It is a functionally optimal design for mainboard layout in PC architecture, although BTX claims to improve on it further.

ATX specifies where the CPU is on the mother board, the size of the PSU, the position of the screw holes in the motherboard, the power connectors, where the holes on the back of the case should be, where the connectors on the front of the motherboard (for the front lights and buttons) should be etc.

While it was an excellent standard when conceived, it has aged. ATX was designed with the excellent idea of modular components, including each being responsible for its own cooling. Unfortunately, modern computers tend to have much more demanding cooling needs than once upon a time, and cooling components individually is a recipe for noisy systems. An integrated approach to cooling that addresses the needs of multiple components, for example using a heatpipe, drastically reduces both the flexibility of the design on the one hand as well the noise on the other hand.