The ship of the line was a square-rigged, three-masted warship that formed the core of most navies from the 17th century to the 19th century. The term "ship of the line" is derived from the principal fighting formation used by navies in this period: the line of battle. This involved opposing fleets forming lines and sailing parallel while firing at each other. As the fleet with the heaviest broadsides usually won, the trend was to build large ships mounting 60 to 110 guns.
Ships of the line were made obsolete in the mid-19th century with the development of ironclad warships. The only ship of the line still afloat in its original condition is HMS Victory which is docked at Portsmouth, England.