Tremolo

A tremolo is a rapid repetition of a single note or chord.

The only way to play a convincing tremolo on the harp is to use enharmonic doubling of the repeated pitch. Two adjacent strings are set to the same pitch by means of the pedals and the tremolo then played in the same way as a trill or a bisbigliando. By using the pedals it is possible to find an enharmonic equivalent to all the notes of the chromatic scale except for D natural, G natural and A natural.
This principal can be applied to single notes or chords.

Repeated notes on one string will not sound like a tremolo. This is because in order to play the string the harpist has to first place the finger on the string, press it and then release. These three separate actions make it very difficult to repeat a note on one string quickly enough to make it sound like a tremolo. 

Furthermore, the resonance of the string will always be damped by the replacing finger. On the other hand, when the tremolo is played on two strings, one string resonates while the other string is damped by the replacing finger, resulting in a continuous sound.

A tremolo produced on two strings set to the same pitch sounds brilliant and quick, especially when played with two hands.

Tremolos must be notated by indicating both enharmonic notes in question.
The harpist will usually start playing a tremolo on the lower string, therefore it is standard practice to notate the lower string first.

When a chord should be played as a tremolo all enharmonic equivalent pitches must be notated.

If a repeated note on one string is the desired effect, standard tremolo notation applies.

For two note tremolos or rapid alterations between two different pitches or chords see: Bisbigliando.