CompTIA A+ Certification All-in-One Exam Guide

Figure 6-2 Typical motherboard
The power supply and the motherboard need matching connectors, and
different form factors define different connections. Given that the term “form
factor” applies to the case, motherboard, and power supply—the three parts
of the PC most responsible for moving air around inside the PC—the form
factor also defines how the air moves around in the case.
To perform motherboard upgrades and provide knowledgeable
recommendations to clients, techs need to know their form factors. The PC
industry has adopted—and dropped—a number of form factors over the years
with such names as AT, ATX, and ITX. Let’s start with the granddaddy of all
PC form factors, AT.