Spanker

The spanker is the large gaff-rigged sail that appeared on the mizzen (rear or aft mast) of nearly all square-rigged European sailing ships during the 18th century and lasted until the end of the age of sail. It was developed to aid sailing before a following wind. It replaced the lateen sail that had previously occupied the aft mast - its advantage being that it did not interfere with the setting of square rig sails on the same mast.