The capital ships of a navy are its most important warships; they are generally the larger ships when compared to other warships in their respective fleet. A capital ship is generally a leading or a primary ship in a naval fleet.
William S. Lind, in the book America Can Win (p. 90), defines a capital ship as follows: "These characteristics define a capital ship: if the capital ships are beaten, the navy is beaten. But if the rest of the navy is beaten, the capital ships can still operate. Another characteristic that defines capital ships is that their main opponent is each other."
There is usually a formal criterion for the classification, but it is a useful concept in naval strategy; for example, it permits comparisons between relative naval strengths in a theatre of operations without the need for considering specific details of tonnage or gun diameters.
First rate: 100 or more guns, typically carried on three or four decks. Four-deckers suffered in rough seas, and the lowest deck could seldom fire except in calm conditions.
Second rate: 90–98 guns.
Third rate: 64 to 80 guns (although 64-gun third-raters were small and not very numerous in any era).
Fourth rate: 46 to 60 guns. By 1756, these ships were acknowledged to be too weak to stand in the line of battle and were relegated to ancillary duties, although they also served in the shallow North Sea and American littorals where larger ships of the line could not sail.