The levers on a lever harp (aka folk harp) are used to adjust the tone of each string--changing it from a natural tone to a flat or sharp note (or, from a flat tone to a natural or sharp). Unlike pedal harps, which range to more than six octaves, lever harps are often much smaller; most have 34 strings (about five octaves), but some are as small as two octaves while some extend to six octaves. The levers are positioned
... close to the top of each string; when it is engaged, it shortens the string so its pitch is raised a semitone... Lever harps are often tuned to the key C or E-flat.
Lever positions can be altered to produce naturals, sharps and flats without having to re-tune strings. Some harps are restricted to only two levers, usually found "on the F and C strings only, allowing a narrower range of keys."