What are some characteristics that define a ATX motherboard?

There are many differences between AT and ATX motherboards. The most noticable, without disassembly, are probably the keyboard connector (AT is much larger) and the power switch. The power switch for an AT motherboard clicks on and clicks off. The power for an ATX motherboard is a momentary push button that is always open unless pressed. Internally the best indicator is the power supply connector to the motherboard. An AT motherboard uses a two piece single row connector. An ATX motherboard connector is one piece with two rows of ten connections.