Flageolet (harmonic)
Overtone sound through light touching of the string.
What it means
With the flageolet (string harmonic), the string is not pressed onto the fingerboard but only lightly touched — exactly at a point that divides the string into equal parts. The fundamental vibration is thereby suppressed, and an overtone sounds: on touching at the middle the octave, at the third the twelfth, at the quarter the double octave. The tone is bright, flute-like, and poor in low partials; hence the name, which goes back to an old flute instrument. One distinguishes natural harmonics on the open string from artificial ones, in which one finger shortens the string and a second rests lightly at the distance of a perfect fourth.
In the notation
Natural harmonics stand as a note with a small circle above it. Artificial ones are written with two noteheads: below, the fingered note as an ordinary head, above it the touching point as a diamond. The sounding note is additionally often given as a small note in brackets, because it does not result at first glance.
Musical use
Harmonics supply a tone colour that sets itself distinctly apart from the ordinary string tone: bodiless, cool, quiet. They appear as closing notes, in floating accompaniment figures, and in the music of the twentieth century as independent material, also in chains or glissandi. On harp and guitar they likewise belong to the fixed stock. Because they have little power, they are accompanied sparingly.
Performance practice
The finger rests lightly, but with clear contact — too much pressure yields a pressed tone, too little a rustling. Decisive is the accuracy of the touching point; practise it first as an ordinarily fingered note to secure the position, and then take the pressure back. The bow needs somewhat more speed at little weight and a contact point rather near the bridge. Artificial harmonics succeed more easily in higher positions, because the fourth turns out smaller there. With a cold instrument or old strings they respond worse — plan for the performance accordingly.