Why does the classic guitar have a body with that specific size and shape?

Question

After having played a steel, cut-out acoustic, and back to my non-cut-out, nylon acoustic, I started to think "what the hell":

• Why so few frets?
• Why the body meets the neck at the 12th fret not higher?
• Why do classic acoustic guitars even have this waist / body shape in the first place?

So the question is: what is the design intent of the shape and position (neck meeting body at 12th fret) of the classic acoustic guitar?

Answer 1

In an acoustic instrument, the tone depends very much on the resonances not only of the material the instrument is made from, but also on the size of the air cavities inside, and also the size and position of the holes connecting the inside to the outside - i.e. the rose on a guitar.

The structure of the instrument also has to be strong enough to withstand the tension of the strings, and any other mistreatment that it might receive while being played or carried around! And of course it has to be physically possible for a "normal sized" human to actually play it.

So in the end, the question "why does a classical guitar look the shape it is" amounts to "why does a guitar sound like a guitar" - which is probably unanswerable! In fact the earliest guitars often had quite small bodies, short necks, fewer strings (e.g only 4) and relatively few frets (sometimes only 7 or 8) compared with the later "Spanish guitar" design.

Of course modern materials and construction techniques can solve the mechanical and acoustic design problems in ways that were not practical using only the materials and hand tools that were available several hundred years ago.

Answer 2

Without any research, whatsoever, my two cents is

(1) The 12th fret is the octave for each string so it is a good visual cue for the player. It may also have to do with the maximum amount of resonance given that that is exactly the half-length of the string.

(2) The Neck is also wider to facilitate the intricacies of the classical repertoire.

(3) There isn't a need for much range beyond the frets for the vast majority of classical guitar music. Of course, some composer push this, but most of the music is fine with the 3 octave range from low E to E on the 12 fret of the first string.

(4) I think the body design with the waist is just for the convenience of putting it on your leg.

Anyways, that's one persons take on it. I'm the interwebs will yield plenty of more info on the topic.