The natural resonances of the strings produce overtones accessible with a light touch.
Place a finger lightly exactly above the 12th fret and pick the string. The resulting tone is a pleasant harmonic an octave above the original open string pitch.
Check out “Roundabout” by Yes for an example of the 12th fret harmonic.
Harmonics occur at other places on the fretboard according to multiple of the string length. The 12th fret harmonic was hallway up the string, and the seventh fret harmonic is 1/3 of the way up the string. These places on the neck that produce harmonics are called nodes. Touching the string lightly on the seventh fret and picking produces a note that is an octave and a fifth above the note. So, the seventh fret harmonic on the A string is an E. Players often use these harmonics to check tuning.
The fifth fret harmonic is ¼ of the way up the string and produces a note two octaves above the fundamental pitch. So, on the E string, the fifth fret harmonic is the same e as the seventh fret harmonic on the A string. Comparing the two notes is a quick check for tuning.
The fourth fret harmonic is 1/5 of the way up the neck. This harmonic is difficult to find at first, but produces a tone two octaves and a major third above the fundamental frequency, a g# on the E string. The same harmonic happens again on the ninth fret, which is 2/5 on the length of the string, and again further up the fretboard.
Check out “Red Barchetta” by Rush for an example of using these harmonics.
The harmonics keep occurring at smaller intervals up the fretboard, but they become more and more difficult to find. The next few are not directly on frets. Look for the next one, another fifth of the scale, about ¼ of the way from the third and fourth fret. The next harmonic, the flat seventh, is about ¾ of the way between the seconds and third frets. (All of these harmonics repeats in many places up the fretboard.) The next harmonic, another octave, occurs about ¼ of the way between the second and third frets. Another harmonic, the second of the scale, occurs at the second fret. Theoretically these harmonics keep occurring on smaller and smaller intervals up the fretboard, but actually playing them becomes more and more difficult.
You may notice that some of these harmonics sound out of tune. When comparing the harmonics with an equally tempered scale, the one usually used in modern music, pitch discrepancies do occur. Scales tuned to the harmonic series actually sound better, but, unfortunately, they only work for that one scale. Every time a key change happened, retuning would be necessary to hold perfect mathematical intonation.
Producing harmonics on fretted notes is also possible. Find the harmonic node with a right-hand finger (usually the index finger) while plucking the string with another finger (usually the ring finger).
For short-distance harmonics, experts use the pinky of the left hand to find the harmonic node.
Tommy Emmanuel combines artificial harmonics with plucked notes to produce an ethereal, harp like effect.
Tapping directly on the fret also produces a harmonic in both natural and artificial harmonics.
Sometimes using the right hand to find the harmonic is called a pinch harmonic. Combining bends, harmonics, and feedback produces a high-pitched squeal. Find an artificial harmonic with the right hand, picking the note then allowing the thumb to lightly touch the string at the node. Bend into that place using high distortion. Listen to “La Grange” by ZZ Top for the definitive squeal sound.