Most lute strings have traditionally been made of animal intestines (gut), metal, or silk, though nylon has become a common replacement for gut. Whatever the material, each string must be of equal thickness throughout its length. Some lutes have only a single string, but the great majority have three, four, or more. Very often there are sets, or courses, of two strings to a pitch, so that an instrument that produces four pitches with open strings actually has eight strings arranged in pairs.