Both the inside faces of the top and back of the guitar have braces for support. The top and back are thin for acoustical reasons and need to be strengthened for structural integrity. The braces are arranged in patterns.
There are many types of bracing patterns used on classical guitars. Almost all of them have produced great sounding instruments. Experimenting with the patterns will teach can teach you a lot about what works and why, but will take a few guitars to learn.
The choice of bracing material depends largely on its physical properties. The size of the braces also has a great effect on how much support they provide. But it is always a balancing act between sound and integrity.
The Purpose of Braces
The primary purpose of braces, both on the sound board and the back of the instrument, is to add structural integrity. A limited tuning range is a by-product not the primary purpose. A top for instance, is under a great deal of tension for the entire life of the instrument. In order to get quality sound out of a top it must be relatively thin. This creates a dilemma. The top is quite thin yet asked to support a great deal of tension. To met the challenge braces are added to the under side of the top and back to give it structural integrity.
Wood for Braces
In order make the top strong without adding an inordinate amount of weight a very light but strong wood is used – spruce. Spruce was used to make planes at one time for this very reason. Its has the best strength to weight ratio of just about any wood. This light weight is important on the top where the mechanical energy of plucking a string is translated to sound by a vibrating plate. Spruce is used for braces regardless of choice of soundboard.
Back braces are typically made of mahogany or Spanish cedar. I have seen then also made of spruce as a well as a few other light but strong woods. Since the back is not under tension like the top is you have more choices. The back braces are mainly for strength, shape and to keep the back rigid.
Braces for the Soundboard
The sound board for a classical guitar is usually a spruce or cedar. These are both soft woods with a high strength-to-weight ratio. This characteristic enables them to be made quite thin and maintain adequate strength while being very light. This also makes them excellent choices for brace wood.
Spruce has the highest strength-to-weight ratio of any wood and is the preferred wood for sound board braces because of that. A brace with the same strength made some other wood will be larger and hence carry more weight making it less ideal.
Braces are always quarter sawn as this is the strongest cut of wood. The grain lines on the end grain are oriented so that they run perpendicular to the surface of the top. Braces are typically cut from a split billet pictured above. The wood has been split along the grain lines to assure it is perfectly quarter sawn with little run out. Brace wood can also be cut on the band saw if you know how to read the grain. If cut this way it may save some wood.
Braces for the Back
For the back of the guitar stiffness and support are the main concerns. The back should be stiffer than the top since it functions as a type of reflector for the vibrating air that is the sound.
Braces are used on the back to make it stiff and stable rather than making the back thicker which would add unnecessary weight to the instrument. The same result can be achieved by adding a few quarter sawn braces. The choice of wood for back braces is not as the same as it is for the top since stiffness is the primary concern an movement is not a consideration.
Typically on a classical guitar back braces are made of mahogany or Spanish cedar. Spanish cedar being the lighter of the two. Mahogany, if you can get your hands of genuine Honduran, is very stiff, stiffer than Spanish cedar and is what I prefer to use on all my guitars. Just as with top braces all back braces are quarter sawn with their end grain lines arranged perpendicular to the surface of the back for maximum strength.