Unlike natural harmonics, which may only be produced at certain string positions, pinch harmonics can be sounded at any fret position along the neck of the guitar. Pinch harmonics also allow the guitarist's fretting hand to stay in position while higher notes than are normally possible at that position are sounded. In addition to expanding the accessible range of pitch, pinch harmonics can be used as unaccompanied tones in a solo or as filler notes between deeper chords.
Pinch harmonics are used extensively in death metal, especially in the sub-genre brutal death metal. The technique's use in death metal is notable in that pinch harmonic notes are included in riffs, rather than being reserved for solos. Combined with the rather low tunings most of these guitarists use, and the fact that they are usually played by both rhythm guitarists (if there are two), the pinch harmonic notes leap out, creating more complex and twisted melodic contours than otherwise possible. The technique is also used commonly in other sub-genres of heavy metal, particularly by guitarists such as K. K. Downing, Zakk Wylde and Dimebag Darrell. One guitarist of the rock genre who is widely known for his use of pinch harmonics is Billy Gibbons of ZZ Top, who uses them numerous times in nearly every guitar solo he plays.
Main article: Overtone
When a guitar string is plucked, the string vibrates at several frequencies. The vibration along the entire length of the string is known as the fundamental, while vibrations occurring between points along the string (known as nodes) are referred to as overtones. The fundamental and overtones, when sounded together, are perceived by the listener as a single tone, though the relative prominence of the frequencies varies among instruments, and contribute to its timbre.
Harmonics are produced on the guitar by lightly touching a string (as opposed to fretting it) at any of several points along its length. The fundamental tone will not vibrate; specific overtones, however, will, resulting in a chimelike tone. Harmonics produced by this method based on open-string fundamentals are termed, natural. If the string is fretted; the harmonics are termed, artificial. Harmonics may only be sounded at the strings' nodes. The nodes for natural harmonics fall at the following points along the guitar's neck:
|1/3||7, 19||octave + fifth|
|1/4||5, 24||2nd octave|
|1/5||4 (3.9), 9, 16||2nd octave + third|
|1/6||3.2||2nd octave + fifth|
|1/7||2.7||2nd octave + minor seventh|
|1/9||2||3rd octave + second|
|1/10||1.8||3rd octave + third|
The technique must be performed at one of the appropriate harmonic nodes for the note to sound. For example, to produce a pinch harmonic which is one octave higher than the fundamental of a string which is stopped at the third fret of a guitar, the string must be plucked halfway between the third fret and the bridge (i.e. 15th fret as the neck 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. the same note in a higher octave) will share the nodes of the lower overtones, so won't be muted. They will, however, be at a much lower volume and since they are the same note in a higher octave, 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 which contains many overtones. For this reason, the gain of an amplified instrument is often increased to make it more easily audible. Thicker strings, stronger pickups and adjustment to amplifier settings (increasing gain) are some ways of doing this. It is important to note that as there is only one fundamental sounding, it will have 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.