On this page about Two-decker:
A two-decker is a sail warship, which carried her guns on two fully-armed decks. Usually additional guns were carried on the upper works (forecastle and quarterdeck) but this was not a continuous battery so were not counted.
In the British Royal Navy, a third-rate was a ship of the line mounting 64 to 80 guns, typically built with two gun decks (thus the related term two-decker). When the rating system was first established, in the 1670s, the third rate was defined as 70 guns, with second-rates having 90 guns, and...
Century, battleship design had settled on a few standard types: older two-deckers (ie. with two... could be used to escort convoys, two-deckers of between 64 and 84 guns which formed the main part of the fleet, and larger three- or even four-deckers with 90-144 guns which were used as admirals...