Truss rod

The truss rod is part of a guitar or other fretted, stringed-instruments that stabilizes the lengthwise forward curvature (also called relief), of the neck. Usually it is a steel bar or rod that runs inside the neck, beneath the fingerboard. Some are non-adjustable, but most modern truss rods have a nut at one or both ends that adjust its tension. The first truss rod patent was applied for by Thaddeus McHugh, an employee of the Gibson company, in 1921, though the idea of a "truss rod" appears in patents as early as 1908.