In the 17th century fleets could consist of almost a hundred ships of various sizes, but by the middle of the 18th century, ship-of-the-line design had settled on a few standard types: older two-deckers (i.e., with two complete decks of guns firing through side ports) of 50 guns (which were too weak for the battle line but could be used to escort convoys), two-deckers of between 64 and 90 guns that formed the main part of the fleet, and larger three- or even four-deckers with 98 to 140 guns that served as admirals' command ships. Fleets consisting of perhaps 10 to 25 of these ships, with their attendant supply ships and scouting and messenger frigates, kept control of the sea lanes for major European naval powers whilst restricting the sea-borne trade of enemies.