The three form factors for desktop system boards are: ATX, and BTX and NLX
Atx, at and btx.
AT, BTX, ATX
The NLX form factor has a riser card on the edge of the motherboard.
The motherboard is located on the form factor
power supply
The case, motherboard, and power supply must all be the same form factor. Obviously the motherboard and the case would have to be the same or the motherboard would never fit in the case. Then the power supply must not only fit in the case, but have the same connectors as the motherboard. Otherwise it won't work.
Case, Power Supply and Motherboard The case must support the form factor of the motherboard, and the motherboard must support the CPU socket type for any given CPU.
It is BTX
BTX
ATX.
ATX
For sure, it is the Motherboard, the Power supply, and the Chassis that determines the form factor of a computer.
The motherboard form factor AT only has the 5-pin AT keyboard connector built in.
xl atx i think