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A ship of the line was a type of naval warship constructed during the Age of Sail from the 17th century to the mid-19th century. The ship of the line was designed for the naval tactic known as the line of battle, which involved the two columns of opposing warships manoeuvering to volley fire with the cannons along their broadsides. In conflicts where opposing ships were both able to fire from their broadsides, the faction with more cannons firing – and therefore more firepower – typically had an advantage.

From the end of the 1840s, the introduction of steam power brought less dependence on the wind in battle and led to the construction of screw-driven wooden-hulled ships of the line; a number of purely sail -powered ships were converted to this propulsion mechanism. However, the rise of the ironclad frigate, starting in 1859, made steam-assisted ships of the line obsolete. The ironclad warship became the ancestor of the 20th-century battleship, whose very designation is itself a contraction of the phrase "ship of the line of battle" or, more colloquially, "battleship of the line".

The term "ship of the line" fell into disuse except in historical contexts, after warships and naval tactics evolved and changed from the mid-19th century. Some other languages did keep the name however; the Imperial German Navy called its battleships Linienschiffe until World War I.