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ATX (Advanced Technology EXtended motherboard) improves on the previous Baby AT & AT form factors, by rotating the orientation of the board 90 degrees. This allowed for a more efficient design, with disk drive cable connectors nearer to the drive bays and the CPU closer to the power supply and cooling fan. ATX is not an abbreviation and is actually a trademark that belongs to Intel.
The AT standard had only a keyboard connector. Other common I/O connectors (serial & parallel ports etc) had to be mounted individually. The ATX was the first computer motherboard to not only include I/O support (serial, parallel, mouse, etc.), but to place all the connectors directly (soldered) onto the motherboard. ATX allowed each motherboard manufacturer to put these ports in a rectangular area on the back of the system, with an arrangement they could define themselves.
