I like comparing the designs of the classic Martins and Gibsons because while I love the sounds of both, they have distinct tonal differences. The tonal differences and the design differences have helped me to form some opinions and understanding of what I can do as a maker to affect the sound of the instruments I build.
Just to be clear, I'll talk about pre war Martin Dreadnaughts, and Early 40's Banner headstock Gibson Dreadnaughts I've gotten to work on and examine. Trying to describe sound is always tricky, but in general I'd say the Martins sound fuller, warmer, and perhaps a bit louder. The Gibsons have more edge, focus, and clarity. Just my opinions, and I'd really rather you listen to as many as you can and formulate your own ideas on their sounds. I'll list the main design differences, and then go into more detail about each of them in future posts.
Body shapes are similar enough that if you take the Gibson sloped shoulder body style and do everything else in a Martin style, it sounds like a Martin. They are both X-braced with 2 lower transverse braces and 2 finger braces off each side of the X-braces.
Gibson 24.625", Martin 25.4"
Arch built in to the top;
Gibsons had quite a bit of arch, about a 20' radius,
Martins were built flat, though over the years string tension has pulled an arch into them
Angle of the X-braces;
Gibsons were around 103 dgrees
Martins 98 degrees
I've seen some variation from these numbers in the guitars I've seen and I'm sure there are more. These guitars were in ways more hand made, though in factories by many pairs of hands, then most of the modern instruments from individual makers. It all depends on your definistion of "hand made", but my point is there was some variation.
Now go play lots of cool old guitars and we'll talk more about all of these differences and others.