Ship of the line

In the age of sail, after the development of the line of battle in the mid 17th century, and up to the mid 19th century, a ship of the line (of battle) was a warship powerful enough to take a place in the battle line. (The term "battleship" has a similar etymology.) Generally, this meant a third-rate or larger ship, with guns on two or three (or in rare cases, four) decks. Rated ships smaller than this were frigates, which mostly carried all their guns on a single deck. European navies in particular used battleships to fight fleet actions which might last for days and involve over 100 ships.

The rating system of the Royal Navy

During the 18th and early 19th centuries, the Royal Navy used the following rating system for its warships:

Type	Rate	Guns	Gun decks	Men	Displacement in tonnes
Ship of the line	1st Rate	100 or more	3 + forecastle and quarterdeck	850 to 875	>2000
	2nd Rate	90 to 98	3 + forecastle and quarterdeck	700 to 750	about 2000
	3rd Rate	64 to 80	2	500 to 650	1300-1600
Frigate	4th Rate	50 to 60	2	320 to 420	about 1000
	5th Rate	32 to 40	1	200 to 300	700 to 1450
	6th Rate	20 to 28	1	140 to 200	450 to 550
Sloops		16 to 18	1	90 to 125	380
Gun-brigs and Cutters		6 to 14	1	5 to 25	<220

The number of crew on a first rate was increased by 25 when used as an Admiral's flagship, by 20 with a Vice Admiral and 15 with a Rear Admiral.

Originally from http://www.cronab.demon.co.uk/gen1.htm, with the author's permission.

In fiction

Isaac Asimov adapted the term "ship of the Line" to apply to the armed spaceships which served a similar role to the old naval vessels for the Galactic Empire of his Foundation trilogy, as mainstays of the space fleet.