A ship-of-the-line was a type of naval warship constructed from the 17th century through the mid-19th century, the culmination of a naval tactic known as the line-of-battle, in which two columns of opposing warships would manoeuvre to bring the greatest weight of broadside guns to bear. Since these engagements were almost invariably won by the heaviest ships carrying the most powerful guns, the natural state of progression was to build the largest, most powerful sailing vessels at the time.
From the end of the 1840s the introduction of steam power led to the construction of screw-driven but wooden-hulled ships-of the line, and a number of pure sail-driven ships were converted to a similar layout. However, the introduction of the ironclad frigate in about 1859 led swiftly to the decline of the steam-assisted ships-of-the-line, with the ironclad warship becoming the ancestors of the battleship.

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Sloop-of-war Origins
The origin of the ship-of-the-line can be found in the great ships built by the English in the 15th and 16th centuries, and the similar large carracks built by other European nations at the same time. These vessels, developed from the cogs, which traded in the North Sea and the Baltic, had an advantage over galleys because they had raised platforms called "castles" at bow and stern which could be occupied by archers, who fired down on enemy ships. Over time these castles became higher and larger, and eventually started to be built into the structure of the ship, increasing overall strength.
Mary Rose was an English carrack and one of the first to be able to fire a full broadside of cannons, being well equipped with 78 guns (91 after a 1536 upgrade). Built in Portsmouth, England (1509–1510) she was thought to be named after King Henry VIII's sister Mary and the rose, the Tudor emblem. She was one of the earliest purpose-built warships to serve in the English Navy; it is thought that she never served as a merchant ship. She displaced 500 tons (700 tons after 1536), was 38.5 m long and 11.7 m abeam and her crew consisted of 200 sailors, 185 soldiers, and 30 gunners. Although she was the pride of the English fleet she was accidentally sunk in an engagement with the French July 19th 1545.
Henri Grâce à Dieu (French, "Henry Grace of God"), nicknamed "Great Harry", was an English great ship of the 16th century. Contemporary with Mary Rose, Henri Grâce à Dieu was 165 feet (50 m) long, weighing 1,000–1,500 tons and having a complement of 700–1,000. It is said that she was ordered by Henry VIII in response to the Scottish ship Michael, launched in 1511. She was originally built at Woolwich Dockyard from 1512 to 1514 and was one of the first vessels to feature gunports and had twenty of the new heavy bronze cannon, allowing for a broadside. In all she mounted 43 heavy guns and 141 light guns. She was the first English two-decker and when launched she was the largest and most powerful warship in Europe, but she saw little action. She was present at the Battle of the Solent against Francis I of France in 1545 (in which Mary Rose sank) but appears to have been more of a diplomatic vessel, sailing on occasion with sails of gold cloth. Indeed, the great ships were almost as well known for their ornamental design (some ships, like the Vasa, were gilded on its stern scrollwork) as they were for the power they possessed.
These ships were the first used in experiments with carrying large-caliber guns aboard. Because of their higher construction and greater load-bearing ability, this type of vessel was better suited to gunpowder weapons than the galley. Because of their development from Atlantic seagoing vessels, the great ships were more weatherly than galleys and better suited to open waters. The lack of oars meant that large crews were unnecessary, making long journeys more feasible. Their disadvantage was that they were entirely reliant on the wind for mobility. Galleys could still overwhelm great ships, especially when there was little wind and they had a numerical advantage, but as great ships increased in size, galleys became less and less useful.
Another detriment was the high forecastle, which interfered with the sailing qualities of the ship; the bow would be forced low into the water while sailing before the wind. But as guns were introduced and gunfire replaced boarding as the primary means of naval combat during the 16th century, the medieval forecastle was no longer needed, and later ships such as the galleon had only a low, one-deck high forecastle. By the time of the 1637 launching of Britain's powerful Sovereign of the Seas, the high forecastle was gone altogether.
From the 16th to 18th century, the great ship and carrack evolved into the galleon, a longer, more maneuverable type of ship, with all the advantages of the great ship. The opposing British and Spanish fleets of the 1588 Spanish Armada were both largely composed of galleons.
With the growing importance of colonies and exploration and the need to maintain trade routes across stormy oceans, galleys and galleasses (a larger, higher type of galley with side-mounted guns, but lower than a galleon) were used less and less, and by about 1750 had little impact upon naval battles. By the 1710s every major naval power was building ships like these.
Large sailing junks of the Chinese Empire, described by various travelers to the East such as Marco Polo and Niccolò Da Conti, and used during the travels of Admiral Zheng He in the early 15th century, were contemporaries of such European vessels, but China never developed them into such advanced fighting ships, and when European interests overtook China the remnants of these sailing junk fleets were vastly outclassed.

Great ships, carracks and galleons
In the early to mid 17th century, new fighting techniques came to be used by several navies, in particular those of England and the Netherlands. Previously battles had usually been fought by great fleets of ships closing with each other and fighting it out in whatever arrangement they found themselves, often using boarding. However, the further development of guns and the adoption of broadside arrangements of guns required a change of tactics. With the broadside the decisive weapon, tactics evolved to ensure as many ships could fire broadside as possible. The line-of-battle tactic required ships to form long single-file lines, and close with the enemy fleet on the same tack, battering the other fleet until one side had had enough and retreated. Any maneouvres would be carried out with the ships remaining in line for mutual protection. Ships considered powerful enough to take a place in the line were known as ships-of-the-line or line-of-battle-ships. Ships too small for the line were stationed out of range of the enemy's guns, to act as scouts and to relay signals between the flagship and the rest of the fleet since, from the flagship, only a small part of the line would be in clear sight.
The adoption of line of battle tactics had consequences for ship design. The height advantage given by the castles fore and aft was reduced, now that hand-to-hand combat was less essential. The need to maneuver in battle made the top-weight of the castles more of a disadvantage. So they shrank, making the ship-of-the-line lighter and more maneuverable than its forebears for the same combat power. As an added consequence, the hull itself grew larger, allowing the size and number of guns to increase as well.

Adoption of the line-of-battle
In the 17th century fleets could consist of almost a hundred ships of various sizes, but by the mid 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 which formed the main part of the fleet, and larger three- or even four-deckers with 98 – 140 guns which were used as admirals' command ships. Fleets consisting of perhaps 10 – 25 of these ships, with their attendant supply ships and scouting and messenger frigates kept control of the sea-lanes for major European naval powers whilst restricting sea-borne trade of enemies.
The most common size of sail battleship was the "74" (so named for having 74 guns), originally developed by France in the 1730s, and later adopted by all battleship navies. Until this time the British had 6 sizes of battleship, and they found that their smaller 50- and 60-gun ships were becoming too small for the battle-line, while their 80s and over were 3-deckers and therefore unwieldy and unstable in heavy seas. Their best were 70-gun 2-deckers of about 150ft long on the gundeck, while the new French 74s were around 170ft, and after Britain captured a few of these French ships in the War of Austrian Succession in the 1740s, Britain's new naval designers from 1755, the joint Surveyors Thomas Slade and William Bately, was able to break away from the past and design several classes of 168ft-170ft 74s, starting with the Dublin and Bellona classes, while their successors gradually improved handling and size through the 1780s. Other navies ended up building 74s also as they had the right balance between offensive power, cost, and manoeuverability. Eventually around half of Britain's ships of the line were 74s. Larger vessels were still built, as command ships, but they were more useful only if they could definitely get close to an enemy, rather than in a battle involving chasing or manoeuvering. The 74 remained the favoured ship until 1811, when Seppings's method of construction enabled bigger ships to be built with more stability.
In a few ships the design was altered long after the ship was launched and in service. In the Royal Navy smaller two-deck 74 or 64 gun ships-of-the-line which could not be used safely in fleet actions had their upper decks removed (or razed), resulting in a very-stout, single gun-deck warship which was called a razee. The resulting razeed ship could be classed as a frigate and was still much stronger. The most successful razeed ship in the Royal Navy was HMS Indefatigable which was commanded by Sir Edward Pellew.
The largest sailing three-decker battleship ever built was the French Valmy, launched in 1847. She had right sides, which increased significantly the space available for upper batteries, but pejorated the stability of the ship; wooden stabilizers were added under the waterline to address the issue. Valmy was thought to be the largest sort of sailing ship possible, as larger dimensions made the maneuver of riggings impractical with mere manpower. She participated to the Crimean War, and after her return to France later housed the French Naval Academy under the name Borda from 1864 to 1890.

Evolution of design in the 17th and 18th centuries
Although Spain, the Netherlands and France built huge fleets, and in France's case with better ships, they were rarely able to match the skill of British naval crews. British crews excelled, in part, because they spent much more time at sea, were generally better fed, were well trained in gunnery (allowing a faster rate of fire), and were generally more competent as the Royal Navy based promotion much more on merit rather than purchase. Traditionally neglecting the British Army, which, historically, has usually been smaller than the armies of comparably prominent continental countries, Britain devoted more resources to her prized navy.
In the North Sea and North Atlantic Ocean the fleets of the Great Britain, the Netherlands, France and Spain fought numerous battles in support of their land armies and to deny the enemy access to trade routes. In the Baltic Sea, Sweden, Denmark, the Netherlands and Russia did likewise, while in the Mediterranean Sea Russia, Ottoman Turkey, Venice, Britain and France battled for its control.
During the Napoleonic Wars, Britain defeated Europe's major naval powers at battles such as at Copenhagen, Cape St. Vincent, Aboukir ("The Nile") and Trafalgar, allowing the Royal Navy to establish itself as the world's primary naval power. Spain, Denmark and Portugal largely stopped building battleships during this time under duress from the British. Britain emerged from the Napoleonic Wars in 1815 with the largest and most professional navy in the world, composed of hundreds of wooden, sail-powered ships of all sizes and classes. The Royal Navy had complete naval supremacy across the world following the Napoleonic Wars, and demonstrated this superiority during the Crimean War in the 1850s.
Nonetheless, the Napoleonic Wars, as well as the American War of 1812 had illustrated the shortcomings of ships-of-the-line when an enemy resorted to tactics including the large scale use of privateers. Both the French and the Americans had demonstrated what a menace small, lightly-armed, but fast, nimble, and, most-especially, numerous vessels like sloops and schooners could be when they spread across the wide oceans, operating singly or in small groups. Targeting the merchant shipping that was Britain's economic lifeblood, ships-of-the-line were too few, too slow, and too clumsy to be employed against them. Overwhelming firepower was of no use if it could not be brought to bear (the Royal Navy's initial response to Napoleon's privateers, which operated from French New World territories, was to buy Bermuda sloops). Similarly the East India Company's merchant vessels became lightly armed and quite competent in combat during this period, operating a convoy system under an armed merchantman.

Combat with ships of the line
The first major change to the ship-of-the-line concept was the introduction of steam power as an auxiliary propulsion system. The first military uses of steamships came in the 1810s, and in the 1820s a number of navies experimented with paddle steamer warships. Their use spread in the 1830s, with paddle-steamer warships participating in conflicts like the First Opium War alongside ships-of-the-line and frigates.

Steam power
During the mid 19th-century the ship-of-the-line was made obsolete by the ironclad warship, a vessel armoured in iron plate and propelled by the steam engine which enabled it to better choose its placement in battle. New guns would simultaneously appear as well, and be carried by both wooden and ironclad ships, but the obsolescence of the ship-of-the-line, indeed of all wooden warships, was not fully realized until March 8, 1862 during the first day of the Battle of Hampton Roads, when two powerful wooden warships were sunk and destroyed outright by the Confederate ironclad CSS Virginia. However, the power implied by the ship-of-the-line would find its way into the ironclad, which would develop during the next few decades into the concept of the modern battleship.

In fiction

Naval tactics in the Age of Sail
Dutch Navy ships of the line
French Navy ships of the line
Royal Navy ships of the line
Spanish Navy ships of the line
United States Navy ships of the line
Regalskeppet Vasa
Man of war