Pinch harmonic example on the 3rd fret of the G string, made on an Ibanez Prestige played through a distortion pedal and played with vibrato
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A pinch harmonic (also known as squelch picking, pick harmonic or squealy) is a guitar technique to achieve artificial harmonics in which the player's thumb or index finger on the picking hand slightly catches the string after it is picked, canceling the fundamental frequency of the string, and letting one of the harmonics dominate. This results in a high pitched sound which is particularly discernible on an electrically amplified guitar as a "squeal".
Use in rock and metal
The technique is possible on any fretted stringed instrument, but is most widely employed by electric guitarists, especially in heavy metal and rock music where heavy distortion ensures that the otherwise subtle harmonic is greatly amplified. An early example can be heard in Roy Buchanan's 1962 recording of "Potato Peeler". Robbie Robertson, who learned the technique from Buchanan, used the technique, as did Leslie West.
A guitarist of the rock genre widely known for his use of pinch harmonics is Billy Gibbons of ZZ Top, who uses them frequently in guitar solos, an example being the second solo on the well-known "La Grange".
Artists such as Eddie Van Halen, Joe Satriani and Steve Vai made the technique popular, utilizing the tremolo arm and high gain amps together with the pinch harmonic to produce horse-like wails from the instrument.
The technique is used commonly in heavy metal, particularly by guitarists such as Criss Oliva, Adam Dutkiewicz, Tommy Victor, Steve Morse, Mick Thomson, Glenn Tipton, John Sykes, Zakk Wylde, Angus Young, Randy Rhoads, Mark Morton, Synyster Gates and Dimebag Darrell. Pinch harmonics are used extensively in death metal, often included in riffs rather than reserved for solos. Combined with the rather low tunings common to the genre, and the fact that they are usually played by both rhythm guitarists (if there are two), the pinched notes leap out, creating more complex and twisted melodic contours than otherwise possible.
A pinch harmonic is produced when the thumb of the picking hand lightly touches against the string immediately after it is picked. This action is sufficient to silence the fundamental and all overtones except those that have a node at that location. This is generally accomplished by holding the guitar pick so that very little of its tip protrudes between the thumb and forefinger (roughly 3–5 mm), allowing the thumb to brush the string immediately after it is picked.
The technique must be performed at one of the appropriate harmonic nodes for the note to sound. For example, to produce a pinch harmonic one octave higher than the fundamental of a string stopped at the third fret of a guitar, the string must be plucked halfway between the third fret and the bridge (i.e., the 15th fret, as fret spacing is logarithmic). Other overtones of the same fundamental note may be produced in the same way at other nodes along the string. The point at which the string is plucked therefore varies depending on the desired note. Most harmonics have several accessible nodes evenly spaced on the string; so it is no surprise that the nodes used in practice are normally those around where the string is normally picked (around the pickups on an electric guitar), rather than those above the neck as these are the easiest to access with the picking hand from normal playing.
Overtones with a frequency of a multiple of the intended overtone (i.e. its own harmonics) will share the nodes of the lower overtone, so they won't be muted. They will, however, be at a much lower volume, and since they form the selected overtone's own harmonic series, they don't detract from the sound of the note. If the string is pinched at the antinode of the intended overtone, no higher overtones will sound.
A single harmonic overtone is far quieter than a normal note with its many overtones. Amplification and related techniques such as distortion or compression are often used to improve the overall sound. Thicker strings, stronger pickups, and adjustment to amplifier settings (increasing gain) are some ways of doing this. Note that as only one of the fundamental sounds, it has a different volume through different pickups, depending on the proximity of nodes or antinodes to the pickup. The different volumes of overtones are the reason pickups sound different. The outcome of this is that if a node is directly over a pickup, it won't sound through that pickup.