A type of join used to afix a headstock to a guitar (or other stringed instrument) neck, or for other woodworking purposes. A scarf joint is typically used to connect two pieces of wood end to end, or with one of the pieces of wood at a slight angle — almost like the wood pieces are interlocking together.
With a scarf joint, the two pieces of wood are cut at complementary angles, then glued together. The angles on the pieces of wood results in a large gluing surface than if the two pieces were simply stuck together in a butt joint. This results in a strong joint (typically the glued joint is stronger than the wood itself) that leaves a barely visible glue line or joint between the two pieces of wood.
For guitar necks, in addition to increased strength, scarf joints mean that less wood required to make the neck and headstock, and pieces of wood that normally wouldn’t be large enough to make a neck can be combined and used.