[Runepune]

Originally Posted by charles Tauber

Guitars have frets that are inserted into slots cut into the fingerboard. Modern frets are "T" shaped and comprised of a "tang" embedded in the slot, which you don't see, except on the ends, if not concealed, and the "crown", which is the more-or-less semi-circular part of the fret that you see and against which the strings are depressed.

The width of the slots are carefully sized to grip the sides of the tang. (Common slots sizes are .021".) If the tang is too thick relative to the slot, the tang wedges the slot "open". If done on several or more frets, the result is that the fingerboard, and neck to which the fingerboard is attached, is bent backwards, creating less relief/neck bow. One controls how much backwards bend occurs by the relative size of the tangs and slots and how many, and where, the oversized-tang frets are placed.

The process is referred to as "compression fretting", and is sometimes used to reduce the amount of forward bow/relief in a neck that doesn't have a functional adjustable truss rod.

Classical guitars usually have higher string action and greater relief than steel string guitars. While too much relief can occur in classical guitars, it isn't particularly common.

Thanks, Charles