In an electrical distribution system power is delivered via three wires, each operating 120 degrees from the other. This is a 3-phase system. As the voltage gets lowered to residential areas, there is often only a single high voltage wire at the top of the poles. This is a single phase from the original system. It feeds a pole mounted transformer which will supply your house with 120/240V. The transformer is center tapped with 120 volts on either side, or you can use 240 volts between the two lines. A generator usually creates electricity by spinning a magnetic field through stationary coils of wire. In three phase systems the coils are placed such that the output voltages are 120 degrees out of phase with each other. This is the same as a three phase electrical system. In order to create the 120/240V system which is commonly used in homes, the generator uses two separate coils. Each coil is 180 degrees out of phase. They are connected in series to create the neutral conductor. This is essentially a two phase system, but is known as single phase, referring to the original electrical system feed which it is designed to replace. By placing the two coils in parallel you can create a higher output 120V system which is single phase.