Gauge is the diameter of a string, a synonym for string-weight.
The tone of a string depends partly on its weight, and, therefore, on its diameter—its gauge. Usually, string diameter is measured in thousandths of an inch (0.001 in = 0.0254 mm). The larger the diameter, the heavier the string. Heavier strings require more tension for the same pitch and are, as a consequence, harder to press down to the fingerboard. A fretted instrument that is restrung with different string gauges may require adjustment to the string height above the frets (the "action") to maintain playing ease or keep the strings from buzzing against the frets. The action height of fretless instruments is also adjusted to suit the string gauge or material, as well as the intended playing style.