A coupe (pronounced "koop-ay", coops are for chickens :)) !) orginally came
from the horse-drawn carriage era, a closed body, with full doors, with only a
single bench seat for 2-3 passengers. This carried over into cars.
In the 1920's, a coupe began to develop the look we associate with it today, a
2-3 passenger car, with side-by-side seating. They came in 3-window (single
side windows on each side, and the back window) and the 5-window, with added
quarter-windows on each side.
By the late 20's, the "club" coupe developed, as a close-coupled closed car,
with the same "bustle trunk", built into the rear of the body, but with a small
back seat, for 2 or 3 passengers. There also developed the "Opera" coupe, with
1 or 2 sideways facing jump seats behind the main seat (AMT's 40 Ford Coupe is
actually an Opera Coupe) The old IMC/Testors 48 Ford Coupe is actually a Club
Coupe.
"Convertible" is a contraction of the old body style name, "Convertible Coupe"
which replaced the original "Cabriolet" name (Cabriolet orginally denoted a
folding top body, with fixed door framing to secure the rolled up window glass,
where the convertible coupe has almost always had fully disappearing side
glass, with no fixed framing above the belt-line
Two-Door sedan bodies are almost always merely a two-door variant of the 4-door
sedan, meaning that the side structure of otherwise identical bodies is the
only difference.
GM complicated the "Coupe" nomenclature with the introduction of the "Hardtop"
in 1949.
GM NEVER made a "hardtop"! They have always catalogued this pillarless 2dr as
a "Sport Coupe".
Today, there aren't very many coupe, or club coupe cars out there, most truly
are 2-dr sedan
(you young guys call them "posts", we old farts knew the difference back
then!--hehehe!)
Hope this long-winded answer clears up this one!