lever harp
As the harp evolved in Celtic lands, it developed a mechanism to more easily produce half tones (sharps and flats). Originally, one had to stop and retune each individual key by using a tuning key to turn the wooden pegs in the top piece of the triangle. When a new mechanism was added, the Celtic harp gained a new name: the lever harp. A lever was depressed onto the string so that the additional pressure was sufficient to raise the tension enough to raise the pitch a halftone (make it sharp). These levers are commonly referred to as sharping levers. This provided more opportunities to change the key within a piece, as Harpers no longer had to take so much time to retune.
"Music Room VI - Romantic Overture"
by Lena Liu
With the advent of the harpsichord, which played on a chromatic scale, the harp’s popularity suddenly dropped. The harpsichord, as an instrument which could play the chromatic scale (included all sharps and flats), was far more versatile than the Celtic harp as one could change keys without any retuning. This spawned the third major development of the harp, the pedal. Unlike the Celtic harpist, the pedal harpist could change keys merely by pressing a pedal. The FAQ’s page at http://www.celticharper.com explains most succinctly:
“The pedal mechanism is fairly complex: pushing the pedals moves discs located at the top of the harp, which perform the same function as levers by shortening (sharpening) the strings. The advantage of pedal harps is that you can play accidentals easily and quickly, as well as being able to change keys quickly.”
From this advance stemmed a great revival of interest in the harp, especially among the noble or wealthy. Although the size of a pedal harp makes it incredibly inconvenient to transport, the fact that it had the ability to play the chromatic scale gave the harp a fighting chance in the struggle for recognition equal to or greater than the harpsichord, the standard parlor instrument of the time. Playing the harp became a fairly common achievement among young ladies of high social station and it also became a staple of large instrumental performances in many Western European countries. The pedal harp, of course, eventually moved to America with the rich people who also migrated to America from the pedal harp’s birthplace in Western Europe. The pedal harp is the harp most people recognize today, as it is now considered to be a staple instrument for symphonies and orchestras worldwide, and is considered especially good at evoking “romantic feeling (Sachs 399).”
Lyon and Healy 40 string pedal harp with spruce soundboard
Gold Pedal Harp