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A motherboard is a printed circuit board that physically holds most of the main components of a computer and provides connections for other components. As a very crude analogy, it could be compared to the skeleton in a human being. Buying a used motherboard can cut PC costs, but can also be a complicated process.
The first thing to check with a used motherboard is the form factor. This is the shape and size of the motherboard. The form factor of the motherboard must be compatible with both the PC casing and the power unit. Fortunately, most modern PCs use the same type of motherboard, known as ATX.
A used motherboard will only be of use if it is compatible with your components. The component most likely to cause trouble here is the CPU or central processing unit. The simplest way round this is to use a motherboard manufactured by the same company as the CPU. Other combinations may also be compatible.
The motherboard must also be compatible with the memory in your PC. This is affected by both the number of pins on the memory unit and whether it uses single or dual in-line memory modules. This compatibility requirement is why most computers are limited in how much memory they can house.
It's also important that the motherboard has adequate connectors and slots. The connectors are there to hook up to the hard drive and there are several types of connection such as IDE and SCSI. If these don't match up to the drive, there will be no way to install and use it.
Some of the key slots to look for are PCI, which is compatible with many types of computer card, and AGP, which is used with most video cards. Other types of card, such as audio or network cards, usually connect to the modern CNR slot. It's also important to check there are USB ports: virtually all new motherboards have them, but some older used motherboards might not, and this would be a serious limitation for a computer.
The wide range of potential incompatibilities with a motherboard is why replacing it in an existing machine can be a troublesome and expensive process. When building a computer from scratch, the situation is slightly different. In this case, whether opting for a new or used motherboard, it's generally best to plan in advance before buying any equipment, making it easier to ensure the motherboard and components are compatible and that there is adequate scope for adding other common components later on.