Question: what are the four most popular types of motherboard form factors?
Answer: The most common form factor is ATX, which evolved to mini-ATX, nano-ATX, pico-ATX, and further. A different type of form factor, and smaller than the ATX is the ITX, which is significantly smaller than the micro-ATX. The ITX form factor can also be found as nano-ITX, pico-ITX, mobile-ITX, and more.