No, that's not the title for a new TV sit-com, but rather a means of minimizing the number of wires needed when building a piece of electronic equipment. Even though all circuits need two wires to operate, the same wires often end up going to a number of different places. In most personal computers, the negative wire coming from the main power supply is used as the common connection, and all other voltages are then referenced to it.

    Many electronic devices are built inside some sort of metal box. Doing this serves not only to provide a solid mechanical structure, but also as a connecting point for any circuit that needs the negative wire from the power supply. As it turns out, a lot of circuits need that connection, so by first connecting the supply wire to the metal chassis, only a short wire from each circuit to the chassis is needed.