The fore-and-aft sail set from the mizzen of square-rigged ships, usually with a gaff and a boom. Although used for propulsion, the principal function of the spanker is to aid the ship in manoeuvre and to be set to assist in the balance of the helm. It grew so large in the big grain carriers that the Germans split it in two with double gaffs so that the two halves could be furled independently. Many square-rigged ships also carry a spanker topsail reaching to the mizzen masthead. In early ships a driver was often used to augment the sail area in light conditions and the term driver is occasionally used in place of spanker.