A three-decker is a sail warship which carried her guns on three fully armed decks. Usually additional guns were carried on the upper works (forecastle and quarterdeck), but this was not a continuous battery and so did not count. Three-deckers were usually "ships of the line

Ship of the line

A ship of the line was a type of naval warship constructed from the 17th through the mid-19th century to take part in the 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...

", i.e. of sufficient strength to participate in the line of battle, and in the rating system of the Royal Navy

Rating system of the Royal Navy

The rating system of the Royal Navy and its predecessors was used by the British Royal Navy between the beginning of the 17th century and the middle of the 19th century to categorise sailing warships, initially classing them according to their assigned complement of men, and later according to the...

 were generally classed as first or second rate, although from the mid-1690s until the 1750s the larger third rates were also three-deckers.