No single gun count defines first rates across the age of sail because the threshold moved as technology and national capacity changed. For the Royal Navy they were major fleet capital ships rather than vessels for routine cruising. Their firepower made them bulwarks of the fighting line, while their size and prestige made them natural flagships. That operational role provides the basis for comparison with later battleship ideas.
Notable Point · 24:19-25:03
What defined a first-rate ship of the line
First rates were a navy's leading fleet capital ships, flagships, and most visible bulwarks of the fighting line.