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 depended on the two columns of opposing warships maneuvering to volley fire with the cannons along their broadsides.
In conflicts where opposing ships were both able to fire from their broadsides, the opponent with more cannons firing – and therefore more firepower – typically had an advantage. Since these engagements were almost invariably won by the heaviest ships carrying more of the most powerful guns, the natural progression was to build sailing vessels that were the largest and most powerful of their time.Watch the video animation from Animagraffs for more info:
In the 17th century fleets could consist of almost a hundred ships of various sizes, but by the middle of the 18th century, ship-of-the-line design had settled on a few standard types.
Older two-deckers i.e., with two complete decks of guns firing through side ports of 50 guns which were too weak for the battle line but could be used to escort convoys, two-deckers of between 64 and 90 guns that formed the main part of the fleet, and larger three- or even four-deckers with 98 to 140 guns that served as admirals’ command ships…source