A brig is a sailing vessel with two square-rigged masts. During the Age of Sail, brigs were seen as fast and maneuverable and were used as both naval warships and merchant vessels. They were especially popular in the 18th and early 19th centuries. Brigs fell out of use with the arrival of the steam ship because they required a relatively large crew for their small size and were difficult to sail into the wind. They are not to be confused with a brigantine, which has different rigging (a brigantine has a gaff-rigged mainsail, while a brig has a square mainsail with an additional gaff-rigged spanker behind the mainsail). A brig is distinct from a three-masted ship by virtue of only having two masts.