a brig is a sailing ship with two square-rigged masts. during the age of sail, brigs were seen as fast and maneuverable. they were used as both naval warships and merchant vessels. even after the sailing age ended they were used as training ships. the brig was one of the smaller two-masted sailing ships. they 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. they were also 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 different from a three-masted ship because it only has two masts.