What determined an Age of Sail ship's rate?

Should an 84-gun two-decker be called a second rate or a third rate?

Its designed gun count makes it a second rate; deck count and gun caliber are not the decisive criteria.

An 84-gun two-decker can still be treated as a second rate even though many familiar second rates had three gun decks. The decisive criterion in the system described here is the number of guns for which the ship is pierced. Deck count and gun calibre do not by themselves move a vessel into another rate. Visual form and inconsistent later labels can mislead order-of-battle comparisons.