Everybody (almost) has their own definitions for many of these terms. These are from
The Distionary of Automotive Terms:
"Coupe
An enclosed single-compartment body with two doors and varying Passenger capacity depending on seat arrangements. The SAE standard J1100 defines it as having less than 33 cubic feet (934 liters) of interior Volume. Larger coupes have rear quarter windows. Coupes have fixed permanent back Panels and top, as well as a luggage compartment in the rear deck. Originally it meant a vehicle which was cut (thus the French coupé) by a glass partition behind the front seats so that the Driver was exposed to the air while those in the back were enclosed.
"Sedan
The term sedan originally described a conveyance seen only in movies today -- a wheelless vehicle for one person, carried on poles by two men, one ahead and one behind. Automakers borrowed the word and applied it to cars with an enclosed four-door body type, permanent back panels, and top with full-width cross seats front and rear, and Passenger capacity from five to seven depending on Wheelbase. Longer-wheelbase models accept extra passengers in fold-down auxiliary seats. Sedans usually have quarter windows in the rear quarter in addition to windows in all four doors. Trunk racks often were standard offerings. Called saloon in U.K. According to Car and Driver, the term "sedan" refers to a fixed-roof car with at least four doors or any fixed-roof two-door car with at least 33 cubic feet (934 liters) of rear interior volume, according to measurements based on SAE standard J1100.
"Roadster
(Rdstr) This term derives from equestrian vocabulary where it was applied to a horse used for riding on the roads. Old dictionaries define the roadster as an open-type car designed for use on ordinary roads, with a single seat for two persons and, often, a rumble seat. It is an open-type body with one bench seat and a luggage compartment in the rear deck. The weatherproof fabric top may be folded, and side curtains may be removed. The Windshield usually may be folded down. The original concept is maintained by the Morgan Plus 8 but modern roadsters include power features such as power steering, electric windows, etc. as in the Mercedes-Benz SL. Also see Sport roadster.
"Touring car
A vehicle with a body longer than the phaeton style, but very similar to it. It permits the use of auxiliary seats in the rear passenger compartment. It was an open car with seats for four or more passengers. Early models had no side weather protection but later were fitted with detachable side screens and curtains. Made until about 1930."
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Most people refer to a small interior, closed 2-door car as a coupe, a large interior car (2- or 4-door) as a sedan (
so long as it has a pillar between the seating positions supporting the roof, if not then it's a hardtop--which essentially didn't exist in 1922), a small interior 2-door open car without wind up windows as a roadster (
If the windows wind up then it's a convertible.), and large interior open car as either a touring car or a phaeton (
The more you paid for it, the more likely the manufacturer and others used the latter term.).