three-decker

thumb|Three-decker HMS Britannia (1820)|Britannia in Portsmouth Harbour, 1835 thumb|Batteries of the 118-gun French ship Océan (1790)|Océan

A three-decker was a sailing warship which carried her principal carriage-mounted guns on three fully armed decks. Usually additional (smaller) guns were carried on the upper works (forecastle and quarterdeck), but this was not a continuous battery and so did not count as a "fourth deck". 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 rates, although from the mid-1690s until the 1750s the larger of the third rates were also three-deckers.