A motherboard form factor specifies the physical dimensions of some of the components of a computer system. It pertains mainly to the motherboard but also specifies compatibility with the computer case and power supply.
The form factor defines the size and layout of components on the motherboard. It also specifies the power outputs from the power supply to the motherboard. The most common form factors are ATX, microATX, and ITX. Let’s discuss these a little further now.

ATX

Advanced Technology Extended (ATX) was originally designed by Intel in the mid-’90s to overcome the limitations of the now-deprecated AT form factor. It has been the standard ever since. The motherboard in Figure 2.1 is ATX. Full-size ATX motherboards measure 12 inches × 9.6 inches (305 mm × 244 mm). ATX motherboards have an integrated port cluster

on the back and normally ship with an I/O plate that snaps into the back of the case, filling the gaps between ports and keeping airflow to a minimum.
One identifying characteristic of ATX is that the RAM slots and expansion bus slots are perpendicular to each other. The ATX specification calls for the power supply to produce +3.3 V, +5 V, +12 V, and –12 V outputs and a 5 V standby output. These are known as “rails” (for example, the +12 V rail). The original ATX specification calls for a 20-pin power connector (often referred to as P1); the newer ATX12V Version 2.x specification
calls for a 24-pin power connector. The additional four pins are rated at +12 V, +3.3 V, +5 V, and ground,