Ship of the line
In the age of sail (up to the late 19th century), a ship of the line was a warship powerful enough to take a place in the line of battle. (The term "battleship" has a similar etymology.) Generally, this meant a third-rate or larger ship, with guns on two or more decks. Rated ships smaller than this were frigates, which carried all their guns on a single deck.
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.