How Can Seven Harp Pedals Retune More Than Forty Strings?

A modern concert harp commonly has forty-seven strings but only seven pedals. The trick is that a pedal does not govern one string; it governs one note name. The C pedal changes every C string across the instrument, while the other pedals do the same for D, E, F, G, A and B. The harp therefore needs only seven strings per octave, with the pedals altering each family together.

Each pedal has flat, natural and sharp positions. Moving it operates rods inside the column and two sets of discs along the neck. Small pins on the discs grip the strings and shorten their vibrating length in semitone steps. This double-action mechanism, developed in the early nineteenth century, gave the diatonically arranged harp practical chromatic reach.

The compromise is equally important: one pedal changes every octave of its note name, so a harpist cannot keep a low C natural while independently sharpening a high C. Composition and pedalling are therefore inseparable. In a glissando, listen beyond the bright sweep. The seven pitches selected beforehand determine whether the gesture sounds like a major scale, a diminished seventh chord or another colour. The pedals are not accessories; they are part of the harp's harmonic language.

https://philharmonia.co.uk/resources/instruments/harp/
https://www.vsl.co.at/academy/strings/harp
https://www.metmuseum.org/art/collection/search/503991