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", i.e. of sufficient strength to participate in the line of battle, and in the rating system of the Royal Navy were generally classed as first or second rate, although from the mid-1690s until the 1750s the larger third rates were also three-deckers.