In the age of sail, after the development of the line of battle tactic in the mid 17th century, and up to the mid 19th century, a ship of the line was a warship powerful enough to take a place in the battle line. Another term, line of battle ship, was shortened to become "battleship", but this is a later usage and is not appropriate for the age of sail.

The fact that a ship would have to stand and fight its opposite number in the enemy line, whatever the size of the enemy ship, put a lower limit on the size of vessel which could be allowed into the line. Generally, this meant a third-rate or larger ship, with guns on two or three, or in rare cases, four decks. Before 1700, the minimum size for a ship of the line would be around fifty guns, steadily climbing to seventy-four during the late Napoleonic period. European navies in particular used ships of the line to fight fleet actions which might last several days and involve over 100 ships.

Ships too small to stand in the line were used for convoy escorts, scouting, patrolling, raiding, blockading, and as flagships on foreign stations. Frigates, which mostly carried all their guns on a single deck, were the most successful all-round design for those purposes.