The outer diameter of a guitar or bass string, always measured in thousandths of an inch. For example, .008″ or .056″ etc. Strings are generally packaged by specific gauges and/or sets of matched gauges. In most cases, guitarists define their preferred string gauge using the number of the highest string. On a standard acoustic or electric guitar, a player might say he or she prefers “nines” or “tens” which means string sets where the highest E string is either .009″ or .010″).