Here you see the so called "split phase" system. This is mainly used in the U.S.A. These one phased transformers (usually fixed to a pole), use a center tap on the secondary coil. This center tap is connected to earth, "splitting" the voltage in equal, but 180° lagged voltages. The voltage between phase and neutral, is 120V (rms). This is used for ordinary devices. For the heavy stuff, they use the 2 phases, which will be a voltage of 240V (rms).
