Fretless guitar

A fretless guitar is a guitar with a fingerboard without frets, typically a standard instrument that has had the frets removed, though some custom-built and commercial fretless guitars are occasionally made. The classic fretless guitar was first pioneered in 1976 by Turkish musician Erkan Oğur. Fretless bass guitars are readily available, with most major guitar manufacturers producing fretless models.

On the fretless guitar, the performer's fingers press the string directly against the fingerboard, as with a violin, resulting in a vibrating string that extends from the bridge (where the strings are attached) to the fingertip instead of to a fret.

Technique
Musicians employ a standard harmony and the twelve-tone technique as a base for exploring tones, using a fretless guitar. Fretless guitars offer musicians an ability to use just intonation in any key and mode and explore new sounds through using microtonal harmonies and folk melodies in a jazz-groove context. A detailed article on extended techniques for fretless electric guitar - written by British guitarist Rich Perks - was published by Music and Practice journal in 2019.

Musicianship with fretless guitars differ from fretted guitars:
 * They require greater finger position precision, because the position of the node of the string is continuously variable (being established by the position of the finger) rather than fixed (established by the position of a fret). As a consequence of this, chordal playing in particular is more difficult to achieve accurately.
 * The string resonance is reduced, requiring more forceful plucking or modified amplification (pickups) to achieve desired volume.
 * The smooth surface of the fingerboard allows legato playing, with smooth transitional slurs between notes

Fretless guitars are uncommon in most forms of western music and generally limited to the electrified instruments, due to their decreased acoustic volume and sustain. The fretless bass guitar has found popularity in many forms of western music, from pop to jazz. The first use of fretless bass guitars dates back to Bill Wyman in the early 1960s. 
 * Maartin Allcock Multi-stringed instrumentalist with Fairport Convention, Jethro Tull and Bully Wee Band. Session Work with Eddi Reader, Robert Plant & Beverley Craven and many others.
 * Rambo Amadeus, a Montenegrin-Serbian social satirist/comedian and experimental jazz/rock musician, among other things, he is known for playing fretless guitar in his performances.
 * Adrian Belew used a fretless guitar extensively on King Crimson's 1984 album Three of a Perfect Pair.
 * Matt Bellamy of Muse now uses a custom Manson double neck with one neck fretless live for two songs.
 * John Cale used a fretless guitar on the 1965 album Stainless Gamelan - a very early recording of fretless guitar.
 * Ned Evett plays a variety of fretless guitars, typically with a glass fingerboard.
 * David Fiuczynski plays fretless guitar extensively in his instrumental project KiF.
 * John Frusciante used a fretless Stratocaster on the recording of Blood Sugar Sex Magik (most notably on the guitar solo of "Mellowship Slinky in B Major"); he now uses custom made fretless guitars with glass fingerboards.
 * Nigel Gavin regularly uses a Godin Glissentar in live performance and for several pieces on his albums Thrum and Visitation.
 * Guthrie Govan plays a Vigier fretless guitar.
 * Chuck Hammer layers multiple tracks of fretless guitars on film scores.
 * Aziz Ibrahim plays Godin and Vigier fretless guitars.
 * Benn Jordan, a.k.a. The Flashbulb, plays a fretless guitar on the track Steel for Pappa from the album Soundtrack to a Vacant Life.
 * Pat Metheny plays a fretless classical guitar on the title track of the album Imaginary Day.
 * Issei Noro has used fretless guitar from professional debut year in 1979, the user of the most famous Japanese guitarist, and most user are using.
 * Erkan Oğur Turkish pioneer of the fretless guitar. Makes nearly all his music with self-made fretless guitars.
 * Yannick Robert plays his Ibanez signature model on "Vaci Utca" and "Dix cordes de nuit".
 * Karl Sanders plays a double necked guitar which has an 11-string fretless setup on the top neck, which he used on many of the tracks on Nile's album Ithyphallic.
 * Elliott Sharp has occasionally used fretless guitars, such as on his 1996 album Sferics.
 * Ron Thal (also known as Bumblefoot) has used fretless guitars extensively.
 * Steve Vai played a triple neck (12-string, 6-string and 6-string fretless) guitar during live shows many years ago.
 * Franck Vigroux plays fretless guitar on Push the triangle's album "repush" and live acts.
 * Vindsval of Blut Aus Nord used fretless guitars on the microtonal MoRT album and other albums.
 * Frank Zappa used fretless guitars on a few albums in the early and mid 1970s.

Festivals featuring live fretless guitar music have been held for several years both in the US and in Europe. In New York, the first NYC Fretless Guitar Festival was held in 2005. In the Netherlands, the Dutch Fretless Guitar Festival has taken place since 2006.