A lever harp is a harp that uses levers to adjust the pitch of one or more strings. They can be either floor or lap instruments. Lever harps represent the mid-point in harp technology.
A non-levered harp is the simplest of all harps. These harps are simply strings stretched across a frame, with either end of the strings attached to pegs. These harps are problematic in that in order to play in more than one key, the player must manually turn the pegs to adjust the pitch of the strings. Some keys, such as G and F, necessitate changing just one string, but other keys, such as Gb, may require changing up to six. Lever harps developed as a way for the player to modify string pitch more quickly, which became necessary as music became more advanced and composers explored different sounds, pitch relationships and progressions.
On a lever harp, a lever attaches to the top of the string. The lever, when opened, shortens the length of the string and thereby raises it by one half step. This means that the player can move pitches back and forth from flat to natural, or that he can move pitches back and forth from natural to sharp. It is not possible to do both and use the lever to achieve all three notes in a pitch class, such as D flat, D natural and D sharp. Even though each note can be treated enharmonically and therefore has two names — for example, A# is the same as Bb — the inability to achieve both sharps and flats is a major disadvantage when it comes to reading music, because the player has to either recognize the enharmonic relationship "on the fly" or re-tune the entire harp by a half step.
In addition to not being able to achieve all three notes in a pitch class, harpists have another limitation with lever harps. The harpist has to use her hand to open or close the lever. When the hand is occupied with this task, it cannot also play. Although a harpist can keep performing with the hand that isn't busy with adjustment — this is not unlike what pianists do when they need to flip a page of their music — doing this can alter the effect of the music temporarily so that the original intent of the composer is briefly lost.
The limitations a lever harp has inspired harp markers to design a new type of harp. The result was the development of pedal harps, which occurred in 1697. The pedals on these harps connected to mechanisms that, similar to the lever, altered the length and pitch of the strings. Harpists control harp pedals with their feet, so both hands can keep playing. Modern pedal harps are designed so that the player can reach all three notes in a pitch class, as well, although the pedal adjusts all octaves and therefore makes it impossible to play, for instance, a G and G# at the same time.
A lever harp can vary considerably in size. Smaller types are lap harps, which can be as small as 2 feet (60.96 cm), but around 3 feet (.91 m) is more common. The largest lever harps can be up to 5 feet (1.52 m) and, similar to pedal harps, sit on the floor.
The sound of a lever harp depends largely on what makes up the strings. Strings may be either natural gut, wire, or nylon. Additionally, lever harps may be called Irish, Celtic or folk harps, depending on the string material. The term "Celtic harp" is most generic, as it can refer to non-levered and levered harps using any of the three major string materials and loosely means any harp developed under Celtic tradition. Irish levered harps have wired strings. Levered folk harps are any levered harp that plays the traditional or popular folk music of a given region, but they usually have gut or nylon strings.