The pieces for the back are chosen and sanded to the approximate thickness before joining.
This makes the join easier to produce, and to then bring the back to its final thickness
after cutting to shape. The back is generally around 2.5mm thick, but this will depend on
the timber and the requirements of the guitar maker for this particular instrument.
The back is then cut to shape and sanded to the required thickness, about 2.5mm. A decorative
strip is then inlaid down the centre join. This is just and extension of what will be the
bindings later on.
It is not necessary and is not found on many guitars. What is important is the addition of
a supporting strip on the inside of the back. This is usually about 25mm wide and 1mm thick
and made from the same material as the top, or a piece of mahogany.
It is usually cross grain for added support and strength along the centre join. The centre
strip is shaped and sanded smooth and then the cross braces are glued to the back. The cross
braces have the same curvature as the finished back. The back is arched and generally has
about a 5mm - 10mm rise in the middle.
Again the amount of arch varies as per the maker's requirements. The arch adds strength and
alters the acoustic behaviour of the back, and thus the final guitar. The braces are usually
made from a timber that is just slightly less dense than the back itself.
The timber for the braces must be stable and strong. The arch in the back is also important
to lessen the chance of the back cracking in times of low relative humidity. As the humidity
drops the back will flatten out. If the back did not have an arch it would most likely crack.
Also if it was made flat, it would have a disturbing appearance. It would appear to be concave
and this would not be acceptable.
