An autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. The voltage source is applied to two taps and the load is connected to two taps one of which is usually a common connection that is also connected to the source. Each tap corresponds to a different source or load voltage. In an autotransformer a portion of the same winding effectively acts as part of both the primary and secondary winding.