Harps produce beautiful sounds when their strings are plucked with fingers. Today it is much easier to play the harp then it was a couple hundred years ago.
 
    Parts of the Harp:
    The sound board (or soundbox) is the resonant part of the instrument. It forms an acute angle with the shoulder which is connected to the neck. The neck is where the tuning forks are located. The forepillar was first used to strengthen the strings, but in modern harps it is hollowed out to contain the rods working the mechanism. Below is a picture of the double-action harp.
 This picture was taken from "Harp." The New Grove Dictionary of Music and Musicians. 1980 ed. It's found on p. 207. It was edited by A. Berry.
    For some time the medieval harp suited people's needs, but as instrumental music became more popular, it's limits were too narrow. People could not play all the notes they wanted to play without much difficulty. Harps had only one scale and to change the tone meant that you lost the use of one hand at that moment. It was a very tedious process to shorten the string by pressing firmly on it. It was also tiresome to twist a crook (a hook)  so that the string would shorten.
 
    Jakob Hochbrucker of Bavaria made the first pedal mechanism. The strings on his harp could now be raised one half-tone higher by pressing the pedal.
 
    In Hochbrucker's harp, a crook had been used to change the tone. This crook sometimes caused a lot of jarring and impediment to the fingers while in use. It also caused the strings to break when tuning. Georges and Jacques Cousineau, a father and son team, had a solution to this problem. They passed each string through a disc with prongs. When the pedal was pressed, these discs would turn and catch on the string and shorten it.
 
This picture was kindly granted to us by Mike Beville. You can see his page at Athene Records.
    In 1810, Sébastien Érard made his double action harp public. His harp was similar to the single-action, in that it had seven pedals and used the motion of a pedal rod to rotate the discs, or forks. The first movement of the pedal produced the first half-tone; the second movement produced the second half-tone. These pedals did not have to be continually held down by the foot. They could be fixed in a notch and then released when needed.

This picture was taken from "Harp." Grove's Dictionary of Music and Musicians. 1966 ed.

  The difference between single and double-action harps is that in the former, the pitch of the strings can only be raised by one half-tone. In the double-action harps, the pitch can be raised by two half-tones. This just means that the harp can play the natural, sharp, and flat versions of each note.  
Érard's double-action harp has a compass of six and a half octaves (for range, see picture).