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The biggest and most important circuit board in a computer is the motherboard;every other part of the computer is connected to it.There is a set of slots near the back edge of the board for plugging in other circuit boards, called adapter cards. These plug-in slots are for things like modems, sound cards, network cards and just about any other add-on feature you can think of. There is a set of traces connecting these slots to each other. A trace goes from Pin 1 of the first slot to Pin 1 of each of the other slots, and other traces connect each of the Pin 2s, each of the Pin 3s, etc. These traces then go on to connect to the memory, CPU, disk drives and other parts of the computer.