Sometimes none, depends on who's using the definition. The original meanings have evolved since carriage & early car days. Different manufacturers will attribute different labels to vehicles of the same configuration, so will uninformed sellers. Best to check a dictionary & the vehicle's data plate.
A couple has a smaller rear floor than a sedan. '49-52 Fords display many combinations. 2 door coupes and 2 door sedans. With or without a B pillar. Without a B pillar is universally referred to as a hardtop. Personally I think it originally differentiated between full metal tops vs when many cars had canvas center sections surrounded by metal.
@@scottpatterson4105 Many thanks for this authoritative explanation. My granddad had a '63 Buick Electra 4-door hardtop. He called it The Big White Hearse. Fantastic automobile.
