Truss rod

The truss rod is a component of a guitar or other stringed instrument that stabilizes the lengthwise forward curvature (also called relief) of the neck. Usually, it is a steel bar or rod that runs through the inside of the neck, beneath the fingerboard. Some are non-adjustable, but most modern truss rods have a nut at one or both ends that adjusts 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.