yes drag, you're right!
A cabriolet IMO is a derivative of a coupe while the roadster only exist with a removable top and doesn't exist as coupe... anyway in reality the difference is quite obvious, roaster are sunny day cars, two seats, sporty, that can have a roof in extreem conditions, whereas cabrio are everyday cars, practical, 4 seats and not necesserily sporty that can remove the top if needed.
If you want a real example, look at porsche:
911 cabrio: when you see it, it's generaly with the top on, 4 seat's, buisness man on it
boxster : generaly open, 2 seats, blond woman on it.
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