In the rating system of the Royal Navy used to categorize sailing warships in the 18th century, a fourth-rate was a ship of the line with 46 to 60 guns mounted. They were phased out of ship of the line service during the French Revolutionary and Napoleonic Wars, as their usefulness was declining; though they were still in service, especially on distant stations such as the East Indies. Fourth-rates took many forms, initially as small two decked warships, later as large frigates razéed from the initial two deck warships, and occasionally even heavily armed merchant ships such as HMS Calcutta.