the motherboard
An older Motherboard, showing Xpac slots, RAM slots and CPU slots.
The motherboard is where all the pieces of the Computer are connected through data cables, and allow your computer to function. The motherboard also affects speed as it can only send data to and fro at a fixed rate. The motherboard also determines your processor type, RAM version (as explained above), and video card that you can use. Along with hardware type, the motherboard also determines the life of your computer, given that it determines what you can put in it. Most gaming motherboards can support the latest in PC tech, including NVMe SSD's, PCI-E 3.0, DDR4 and the latest in processors from AMD and Intel. Without this crucial piece, the computer would simply not function. While motherboards get fancier and faster, their general layout does not change.
the case
HP Pavilion a1350n computer case (2005)
The case is just as important as any other computer piece. Not only does the case hold the motherboard, and the other computer pieces; but it also determines how many hard drives you can hold, what kind of cooling system you have, and the size motherboard you are able to put in it. If your case doesn't support a certain number of things, or a certain size then you have to buy a new case with the required features. There are also many different size Cases. going from Mini ITX to Full ATX Cases Mini ITX are very small, and can only fit Mini ITX motherboards. Micro ATX are bigger, but still small, and only fit Micro ATX motherboards. Mid ATX (or ATX) are the standard size and can fit Standard ATX and Micro ATX motherboards. Full ATX towers can fit Micro, Standard and Extended ATX Motherboards.