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A spanker is either of two kinds of sail.
On a square rigged ship, the spanker is a gaff-rigged fore-and-aft sail set from and aft of the aftmost mast. Almost all square rigs with more than one mast have one or two spankers, which evolved from the driver sail. Some also carry a triangular topsail above the uppermost or only spanker, between the gaff and the mast, called the gaff sail. A spanker in this situation is often 'soft footed' in that it has no boom to which it is attached at its foot.
The term was also briefly used commercially for a branded type of spinnaker.