A Computer casing is an enclosure or a shell that houses the internal components of a Personal Computer (PC or RIG). Aside from Computer case, there are lots of other names that may be used to refer to it as well, such as 'Case', 'Chassis', 'Enclosure' or 'housing' depending on the preference of the individual that is describing it. This is sometimes wrongly indicated as the CPU, especially by the general public.

There are many types of PC chassis available to users which may be categorised in a few different ways which is normally by the size of the largest Motherboard that it is able to house.

Recently though, this has gotten quite mixed up as there are more different sizes of motherboards than there were a few years back thanks to technological advancements. Having said that, the motherboard isn't the only component that affected the size of the PC chassis, more often than not you'll see chassis that are deeper or longer than normal even though it could only fit a normal ATX-sized motherboard; this is due to several factors such as Longer Power Supply Units, Bigger GPU, Taller CPU, User feedbacks or even manufacturer's naming schemes (such as Ultra-Tower named by Cooler Master to the CosmosII).

Arguably, there are so many things to consider when referring the size of a PC chassis. So in this thread, to save a lot of hassle, here is a general guide on the PC Chassis sizes:

Full-Tower - The biggest beast of them all! Normally with more than 8 expansion slots and is able to house motherboards larger than a normal full-sized-ATX motherboards up to large consumer-grade motherboards such as the EVGA SR2 without (or with minor) modifications

Mid-Tower - The most used size by consumers, has about 7 to 8 expansion slots that houses Full-sized-ATX motherboards or even the slightly larger BTX or E-ATX depending on the design.

Mini-Tower / Small Form Factor (SFF) - Compact chassis that houses Micro-ATX or smaller motherboards that has 4 or less expansion slots