Gut strings – sometimes called catgut – are hand made from the intestines of animals such as sheep or cows. All stringed instruments, including the guitar, were strung with animal gut or silk strings until the advent of nylon in the 1940s.

The three treble guitar strings are made from sheep or cow intestine, referred to as plain gut, while the three bass strings are made of a silk thread core wound with gut.

Gut strings are the natural choice for many classical and baroque string players, and gut strings are still most commonly preferred in concert-tension pedal/grand and some lever harps because they give a richer, darker sound as well as withstanding high tension within low alto, tenor, and high-bass ranges.

Classical guitar strings once made of gut are now made of materials such as nylon or fluoropolymers, typically with silver-plated copper fine wire wound about the acoustically lower (d-A-E in standard tuning) strings.