An autotransformer is a transformer with only one winding, which is wrapped around a set of core pieces. This winding acts both as the primary winding and the secondary winding of the transformer. Instead of having separate primary and secondary windings like a standard two-winding transformer, an autotransformer has only one winding and the voltage is stepped up or down as required. Autotransformers are generally much lighter, less expensive, and more space efficient than two-winding voltage transformers. They are commonly used to provide voltage adjustment in electric power distribution networks, and for things such as impedance matching for antenna designs and audio applications.