Sedans originally had a framed glass, so 2 or 4 door...(ie: a 57 Chevy 210 was a sedan, 2 or 4 dr)...a coupe technically has little or no back seat. Then there Hardtops ( originally called hardtop convertibles)...hardtops are not sedans....wanna get into roadsters?....good luck with that...lol John Miller