If the action on an instrument with a set neck is high and the saddle at the bridge is low, the instrument may need a neck reset to bring it back to its optimal playability. To do this, I remove the neck using heat, cut the heel to the correct angle, refit the joint, and glue it back with hot hide glue restoring proper geometry and playability.
Neck Resets
1970 Martin D-18
Martin used some tough glue during this era but I was still able to get the neck off cleanly, correct the pitch & glue it back in with luthier-friendly hot hide glue. Notice my attention to detail with the rosewood shim at the tongue of the fingerboard. In many cases after correcting the pitch of the neck a shim must be made to avoid a dramatic drop off of the fretboard as it extends over the body.