The Navy's rating system was later modified to differentiate ships considered suitable for various functions within the naval tactics of the age of sail. Lower numbers indicated larger and more capable ships. By the mid-18th century ships suitable for the line of battle were first-rate ships carrying at least 100 guns, second-rate ships carrying 84 to 98 guns, and larger third-rate ships carrying 70 to 80 guns. Smaller third-rate ships carrying about 60–64 guns, and fourth-rate ships of around 50 guns, had earlier been considered suitable, but were being phased out. Fifth-rate and sixth-rate ships were frigates usually maneuvering independently of the line of battle.