From A Detailed Description of an Early 17th Century Italian Five-Course Guitar
Tom and Mary Anne Evans, Guitars - From Renaissance to Rock, 1977
In making the body and neck of a classical guitar, the most complicated joint used is a scarf joint. The scarf joint is used to connect the headstock to the neck shaft, some makers use a more complicated "V" joint to connect the headstock to the shaft. Miter and butt joints are used on the bindings, but this is purely for decoration, bindings are used to cover simple joints. The guitar sides usually fit into slots cut into the heel block, I like to cut a wider, angled slot and use wedges to hold the sides in the heel block.
Anyone who has made a classical guitar with the help of the book, Making Master Guitars, by Roy Courtnall, should recognize this wedged joint. In Making Master Guitars the joint is touted by the master guitar maker, Jose Romanillos, he used this joint and a variation of it until he retired from making guitars.
I began using this joint early on in my journey in guitar making, it made sense. It is a strong joint and unlike cutting a narrow slot, it allows me some wiggle room in fixing how the side fits against the heel and the wedge against the side.
The wider slot allows me to clean up the saw cut that will be seen once the side is attached with a sanding stick, there is no need to see a gap between the side and the heel!
Once the wedge is cut, I put it in the slot with a "dummy"piece of wood that is the same thickness as the side. I then start to cut a kerf where the wedge and the heel block meet...
and continue to "saw kerf joint" the surfaces until...
I have a nice looking joint!
When the side is ready to be attached to the guitar top, all I need to do is to trim the wedge a little short so when I hammer it in the endow the wedge will be just shy of seating against the top. There is no need to glue the wedge in, it is a strong joint and the wedge won't go anywhere. If the wedge is glued then the joint is not reversible, a consideration if the guitar needs to be repaired!