On a square rigged ship, the spanker is a gaff rigged fore-and-aft sail set from and aft of the aftmost mast. More often now called a Mizzen or a Jigger. (In old books you may also find the same sail called a ‘driver’.) The sail which the Dutch sometimes set from masthead to stern, and which they call an aap (ape), could well be called spanker in English.