Unless you’re a really stern system builder, chances are that when you think about your desktop computer, you don’t think too much about the PC Case. In fact, as long as it meets a few basic requirements, everything else is probably pretty much okay: Does it hold all your hardware? Do the buttons and lights work? Are there USB ports on the front? And when it comes right down to it, these aren’t minor concerns. More than any other component, the case is so simple that you only have to look at it to know how it works.

That doesn’t mean, however, that there aren’t plenty of places to go wrong when deciding which PC Case you want to house your new computer (or the components you already have). Accepting what the case does, how it works, and why it’s designed the way it is will help you make smart buying choices. Will choosing the wrong PC Case terminate your computing experience? Probably not. But the right case has the potential to make a mediocre system okay and a good system great. So why take the chance?

Pick the Right Size and Shape

Unlike with motherboards and processors, a case’s size doesn’t need to be explained. Bigger PC Cases can hold more optical and hard drives (see below), have room for bigger video cards, and are easier to work in. The terms “full tower,” “mid-tower,” and “minitower” are sometimes employed to suggest PC Cases’ decreasing size, but if you’re not familiar with lots of different kinds of cases, they won’t mean much to you. Buy what you need for the hardware you have, and what you expect to need later. But you’ll need to give some thought to the issue of form factor. This refers to the design of the PC Case’s interior, and to the type of motherboard the case supports. Though you’ll infrequently see a few others, the most common right now are ATX (which measures about 12 by 9.6 inches) and microATX (which measures about 9.6 by 9.6 inches).

A microATX motherboard will usually work in a case designed around the ATX form factor, but not vice-versa—even if you can somehow cram an ATX motherboard into a microATX case, it will be so tight you won’t have room to do anything else. And because ATX motherboards have more expansion slots, there won’t be any way to access those from outside the case once you’re done building the system. Make sure the motherboard’s form factor matches that of the case, and you’ll never go wrong.

Get Enough Drive Bays

As mentioned above, the larger the case, the more drive bays it’s likely to have. There are three basic sizes of drive bays that determine what kinds of drives can be used in them. The first is the 5.25-inch bay, which is the wider style used for DVD and Blu-ray optical drives. (Some other kinds of hardware, such as specialized fan or audio controllers, are also sized for 5.25-inch bays.) Then there are 3.5-inch bays, which may be either internal or external: For ages, the external version was used for 3.5-inch floppy disks, but now you’ll usually see card readers in those spaces; internal bays are where you’ll put your hard drives. Most full-size tower cases will have two or three 5.25-inch bays and four or five internal 3.5-inch bays; some cases come with specially designed adapters that let you install a 3.5-inch drive in a 5.25-inch bay if your case doesn’t have a 3.5-inch external door on the front. The third kind is the 2.5-inch bay, which is still uncommon in most desktop PCs but it’s slowly becoming more popular. These are intended for smaller hard drives and “naked” solid-state drives (SSDs), and are usually found below the internal 3.5-inch drive well, or perhaps elsewhere on the floor of the case. A few cases do have external 2.5-inch bays, but these are very rare—most of the time, the case will provide some method of securing a 2.5-inch drive in a 3.5-inch bay.

Check Your Expansion Slots

Most ATX cases have six or seven slots on the rear panel for adding expansion cards; microATX cases usually have four. We have seen a few cases that buck these trends—high-end gaming cases designed for users of multiple two-slot video cards can have as many as 10 slots!—but those are rare examples you probably won’t come across accidentally. Check the specs before you buy, just in case