Ship ratings changed as warships grew

The rating system changed during the later age of sail rather than preserving Pepys's seventeenth-century ranges. First and second rates now begin at much higher gun totals, while third rates occupy the familiar 64-to-80-gun range. Fourth rates remain ships of the line at the upper end but can be described as frigates below roughly 60 guns. Fifth and sixth rates likewise shift upward as warships become larger and more heavily armed. An 18-gun vessel that once fitted a rated category can therefore fall below frigate status in the later system. Classification was a moving institutional standard tied to fleet development.