Harmonics (flageolet, sons harmoniques) are "sets of musical notes whose frequencies are related by simple whole number ratios."[OldhamGroveMusicOnline]
The harmonic is a popular effect in harp music. Although the octave harmonic is the most common harmonic in the harp repertoire, fifth, double octave and third harmonics can also be produced. On some strings, even more harmonics can be produced. Harmonics can be used in combination with normal notes as well as with other harmonics. On the harp, as many as four harmonics can be produced simultaneously.
N.B. All harmonics should ALWAYS be notated where they are played.
In earlier harp literature harmonics were notated in two different ways: either where they are played or where they sound. This double practice has caused considerable confusion and discussion in cases where it is not clear which of the two methods is followed. Today, however, almost all notation manuals and harpists recommend that harmonics be written where they are played. The reason is that when the harmonic is notated as played, the harpist can locate the note and execute it much faster. The harpist's eyes cannot be on the music and on the strings at the same time, and therefore it is very convenient when the harpist can automatically locate on the strings symbols notated on the page.