A common wire is either a connecting wire or a type of neutral wiring, depending on the electrical circuit. When it works as a connecting wire, the wire connects at least two wires of a circuit together.
Standard wiring design in U.S. homes involves a neutral wire at the ground potential and two hot wires of 120 volts each. One of the hot wires swings negative and the other one swings positive. A transformer supplies the house and either of the two hot wires can provide electricity to the different 120-volt standard circuits that are in homes. People can combine both wires when a 240-volt circuit is necessary.