There are two types of harmonics, those produced on an open string called natural harmonics, and those produced on a fretted string, called artificial harmonics.
A harmonic is produced by touching a vibrating string lightly at a point of exact division (1/2, 1/3, 1/4, etc.) forming nodes, or points along the string which are stationary or free of vibration. A harmonic played at the 12th fret forms a node which divides the vibrations of the string into halves. This produces a pitch exactly one octave above the open string—the same as the string fingered at the 12th fret.
Harmonics are generally most easily begun on ⑥ at XII:
- With a L.H. finger, lightly touch the string directly above the XII fret, without the string touching the fret.
- Sound the string firmly with p midway between the edge of the soundhole and the bridge.
- Remove the L.H. finger as soon as the harmonic begins to sound.
There is a tendency to muffle the harmonic by touching the string too long, or to sound an open string by removing the finger too soon. Experimenting with how long to leave the finger on the string will yield the most effective timing for producing harmonics.
As the number of string divisions increases the pitch will rise. A harmonic produced at XII produces a pitch exactly one octave higher than the open string. The next higher harmonic, formed at either VII or XIX produces a pitch a perfect fifth higher than the previous octave.
The next harmonic is found at the V fret (or where the XXIV fret would be if the fingerboard were extended) and produces a pitch two octaves higher than the open string. The highest harmonic commonly used on the open strings is found on the IV, IX, or XVI frets. It produces a pitch two octaves plus a major third above the open string. Occasionally higher harmonics are used, but they generally produce such weak tones that they are considered impractical.
There is no standard system for the notation of harmonics in guitar music; the most common approach is to write the abbreviation ‘Harm’ or ‘arm’ (for the Spanish Armonicos) and the fret number above or below an open string. Occasionally a composer (e.g. H. Villa-Lobos) will employ a system similar to that commonly used to notate harmonics in violin music. Diamond shaped notes are written according to the fret and string involved (not the actual pitch of the harmonic), and regular notes are written at actual pitch.
Harmonics on the lower strings are usually sounded with p; those on the higher string with the fingers. But this is entirely a matter of convenience and it should be understood that any finger may be used to play any string whenever advantageous.
Very high harmonics are more distinct and are more readily produced when sounded near the bridge, and struck with considerable force.
Another technique for sounding natural harmonics involves use of the right hand alone: i is extended to lightly touch the string above the desired fret, and a (or p) sounds the string with a F.S.
Artificial harmonics are those which are not formed using an open string, and are always executed with the R.H. harmonic technique. A note is formed as usual with the L.H. and a R.H. harmonic is produced at a fret determined by the location of the L.H. finger. For example, since 12 frets constitute an octave, if Fa on ① is fretted, the R.H. harmonic would be produced at the XII+I or XIII fret. This works for all of the natural harmonic fret distances (12, 7, 19, 5, etc).