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).
In sailing, a full-rigged brig is a vessel with two square rigged masts (fore and main). The main mast of a brig is the aft one. To improve maneuverability, the mainmast carries a small (gaff rigged) fore-and-aft sail.
Brig sails are named after the masts to which they are attached: the mainsail; above that the main topsail; above that the main topgallant sail; and occasionally a very small sail, called the royal, is above that. Behind the main sail there is a small fore-and-aft sail called the spanker or boom mainsail (it is somewhat similar to the main sail of a schooner). On the foremast is a similar sail, called the trysail. Attached to the respective yards of square-rigged ships are smaller spars, which can be extended, thus lengthening the yard, thus receiving an additional sailing wing on each side. These are called studding sails, and are used with fair and light wind only. The wings are named after the sails to which they are fastened, i.e. the main studding sails, main top studding sails, and the main top gallant studding sails, etc.
The brig’s foremast is smaller than the main mast. The fore mast holds a fore sail, fore top sail, fore top gallant sail, and fore royal. Between the fore mast and the bowsprit are the fore staysail, jib, and flying jib. All the yards are manipulated by a complicated arrangement of cordage named the running rigging. This is opposed to the standing rigging which is fixed, and keeps mast and other objects rigid.
A brig is "generally built on a larger scale than a schooner, and may approach the magnitude of a full-sized, three-masted ship." Brigs vary in length between 75 and 165 ft (23–50 m) with tonnages up to 480. A notable exception being the famous designer Colin Mudie's 'Little Brigs' (TS Bob Allen and TS Caroline Allen), which are only 10m long and weigh only 8 tonnes. Historically, most brigs were made of wood, although some later brigs were built with hulls and masts of steel or iron. A brig made of pine in the 19th century was designed to last for about twenty years (many lasted longer).
The word "brig" has been used in the past as an abbreviation of brigantine (which is the name for a two-masted vessel with foremast fully square rigged and her mainmast rigged with both a fore-and-aft mainsail, square topsails and possibly topgallant sails). The brig actually developed as a variant of the brigantine. Re-rigging a brigantine with two square-rigged masts instead of one gave it greater sailing power. The square-rigged brig's advantage over the fore-and-aft rigged brigantine was "that the sails, being smaller and more numerous, are more easily managed, and require fewer men or 'hands' to work them." The variant was so popular that the term "brig" came to exclusively signify a ship with this type of rigging. By the 17th century the British Royal Navy defined "brig" as having two square rigged masts.
Brigs were used as small warships carrying about 10 to 18 guns. Due to their speed and maneuverability they were popular among pirates (though they were rare among American and Caribbean pirates). While their use stretches back before the 17th century, one of the most famous periods for the brig was during the 19th century when they were involved in famous naval battles such as the Battle of Lake Erie. In the early 19th century the brig was a standard cargo ship. It was seen as "fast and well sailing", but required a large crew to handle its rigging. While brigs could not sail into the wind as easily as fore-and-aft–rigged vessels such as schooners, a trait that is common to all square-rigged ships, a skilled brig captain could "manoeuvre it with ease and elegance; a brig could for instance turn around almost on the spot". A brig's square-rig also had the advantage over a fore-and-aft–rigged vessel when travelling offshore, in the trade winds, where vessels sailed down wind for extended distances and where "the danger of a sudden jibe was the large schooner-captain's nightmare". This trait later led to the evolution of the barquentine. The need for large crews in relation to their relatively small size led to the decline of the production of brigs. They were replaced in commercial traffic by gaffsail schooners (which needed fewer personnel) and steam boats (which did not have the windward performance problems of square rigged ships).
The Telos, built in Bangor, Maine in 1883, was reportedly the last brig to join the American merchant marine, and was "considered to be the finest vessel of her class ever constructed in Maine". She was wrecked on Aves Island, off Bonaire in the Caribbean, in 1900.The brig USS Argus used during the First Barbary War and the War of 1812.
Archer, a vessel of the Second Texas Navy.
The brig USS Oneida used during the War of 1812. James Fenimore Cooper was a midshipman aboard the Oneida while under construction.
The cargo hauling brig Farmer owned by George Washington.
The cargo hauling brig Fleetwing.
The brig Leonora of Captain Bully Hayes.
The brig Mercury (Russian: Меркурий) A 1819 Russian navy 18 gun brig painted twice by Ivan Aivazovsky.
The brig USS Niagara captained by commander Oliver Hazard Perry in the Battle of Lake Erie, a pivotal victory for the United States in the War of 1812.
The brig USS Oregon used in the U.S. Exploring Expedition.
The cargo brig Pilgrim, whose 1834 trading voyage from Boston, Massachusetts to California is described in the book Two Years Before the Mast.
Potomac, a vessel of the Second Texas Navy
The brig Rebecca captained by Robert Jenkins whose boarding triggered the War of Jenkins' Ear.
The brig USS Reprisal that fought in the American Revolution.
The brig USS Somers, sunk in the Mexican–American War.
HMS Beagle was built as a brig (1820) for the Royal Navy. She was deployed as a survey vessel to survey the coasts of South America, Australia, and Africa. A mizzen mast was added prior to the 5-year voyage with Charles Darwin to increase the manoeuvrability in the shallow coastal waters that she would explore.
Jean Lafitte's pirate brig, the Pride from 1815 to 1816.
The brig HMS Badger, was the future Admiral Horatio Nelson's first command as a young lieutenant.
Wharton, one of the vessels of the Second Texas Navy, which participated in the Naval Battle of Campeche, which is the only historical example of a steam navy having been defeated by a sail navy.
Joel Root as supercargo sailed out of New Haven harbor in 1802 on the brig Huron to begin his journey around the world on a sealing expedition.
Rover was a privateer brig out of Liverpool, Nova Scotia known for several bold battles in the Napoleonic Wars.
Note that while the famous mystery ship Mary Celeste is sometimes called a brig, she was clearly a brigantine.The 18-gun brig Rattlesnake commanded by Commander Terence O'Brien in Frederick Marryat's Peter Simple.
The brig Lightning in Joseph Conrad's The Rescue and the brig "Bonito" in Conrad's "Freya of the Seven Isles".
The brig Sea Hawk in The Pirate of the Mediterranean by William Henry Giles Kingston.
The brig Interceptor in the film Pirates of the Caribbean: The Curse of the Black Pearl (portrayed by the brig Lady Washington).
The brig Enterprise in the film Star Trek Generations (portrayed by the brig Lady Washington).
The brigs Porta Coeli and Amélie appear in the Horatio Hornblower series by C. S. Forester (which was later adapted to films and television).
The brig HMS Sophie in Master and Commander by Patrick O'Brian.
The brig Triton in Ramage and the Freebooters and Governor Ramage R.N. by Dudley Pope.
The brig Molly Swash, in James Fenimore Cooper’s book Jack Tier.
The brig Hellebore in the Nathaniel Drinkwater series by Richard Woodman.
The brig Isle of Skye in Iain Lawrence's The Wreckers (High Seas Trilogy).
The brig Seahawk in Avi's novel The True Confessions of Charlotte Doyle.
The brig Blue Bird in Evert Taube's song "Balladen om briggen Blue Bird av Hull".
The brig Grampus in Edgar Allan Poe's novel The Narrative of Arthur Gordon Pym of Nantucket.
The brig Jolly Roger, a pirate ship of Captain Hook from James M. Barrie's Peter Pan.
The brig Speedy a pirate ship from Jules Verne's The Mysterious Island.
The brig Constanzia from Jules Verne's A Drama in Mexico.
The brig Forward from Jules Verne's The Adventures of Captain Hatteras.
The brig Duncan from Jules Verne's In Search of the Castaways.
The brig Arkham in H. P. Lovecraft's At the Mountains of Madness.
The brig Poison Orchid in Scott Lynch's Red Seas Under Red Skies.
The brig Covenant in Robert Louis Stevenson's Kidnapped (novel).
The brig HMS Wolverine in L.A. Meyer's Under the Jolly Roger.
The brig Aquila in the game Assassin's Creed III.
The brig Jackdaw in the game Assassin's Creed IV: Black Flag.
The brig-sloop Morrigan in the game Assassin's Creed Rogue.
The Brig 'Beneficence' in Gary R. Bush's "Sail Into Treachery"The brig Lady Washington
The brig Tre Kronor
The brig Niagara
The brig Morgenster
The brig Lady Nelson
The brig TS Royalist
The brig Pilgrim
The brigs Stavros S Niarchos and Prince William
The brig Roald Amundsen 
The brig La Grace
The 'Little Brigs' TS Bob Allen and TS Caroline Allen