Georgia State University's website offers a schematic illustrating both 240-volt and 120-volt household electrical wiring. The standard wiring design in the U.S. is two 120-volt "hot" wires and a neutral ground wire with a total maximum available voltage of 240.
Normally the two 120-volt wires are grounded by the neutral wire at the center tap of the transformer supplying electricity to the house, allowing one hot wire to swing positive and the other negative. This "out of phase" configuration allows for the standard 120-volt household circuit to be supplied by either wire. A 240-volt circuit requires the use of both hot wires.