Detuners or "Xtenders" are mechanical devices used to simplify the tuning

Musical tuning

In music, there are two common meanings for tuning:* Tuning practice, the act of tuning an instrument or voice.* Tuning systems, the various systems of pitches used to tune an instrument, and their theoretical bases.-Tuning practice:...

 of a stringed instrument during performance. This allows the musician

Musician

A musician is an artist who plays a musical instrument. It may or may not be the person's profession. Musicians can be classified by their roles in performing music and writing music.Also....* A person who makes music a profession....

 to quickly and accurately reach notes outside the normal range of their instrument. These devices are also known by other names including 'drop head' and 'hipshot'.

They can be fitted at either or both ends of any or all of the strings. For example, a relatively common arrangement on the bass guitar

Bass guitar

The bass guitar is a stringed instrument played primarily with the fingers or thumb , or by using a pick....

 is to use a detuner on the lowest string to allow the bassist

Bassist

A bass player, or bassist is a musician who plays a bass instrument such as a double bass, bass guitar, keyboard bass or a low brass instrument such as a tuba or sousaphone. Different musical genres tend to be associated with one or more of these instruments...

 to switch between 'standard tuning' (E A D G) and 'drop D' (D A D G). The latter provides two extra notes (D and D# / Eb) that are particularly useful in several common keys

Key (music)

In music theory, the term key is used in many different and sometimes contradictory ways. A common use is to speak of music as being "in" a specific key, such as in the key of C major or in the key of F-sharp. Sometimes the terms "major" or "minor" are appended, as in the key of A minor or in the...

.

However, there are many possible variants - for example, bassist Michael Manring

Michael Manring

Michael Manring is an American electric bassist from the San Francisco Bay Area, .-Biography:...

 has made extensive use of detuners in some of his compositions and has a four-stringed bass with multiple detuners. Adrian Legg

Adrian Legg

Adrian Legg is an English guitar player who has been called "impossible to categorize". He plays custom guitars that are a hybrid of electric and acoustic, and his fingerstyle picking technique has been acknowledged by the readers of Guitar Player who voted Legg the "best acoustic fingerstyle"...

is a popular guitarist making use of rapid tuning changes. He was prominent in the late 1980s.

The idea may have originated from the double bass extenders.