A double harp is a chordophone instrument played by plucking the strings with the fingertips. More specifically, harps normally have only one set of strings. These are called single-course harps. When a harp has more than one set of strings, it is called a multi-course harp. Having two sets of strings, a double harp is a type of multi-course harp.
In terms of tuning, each set of strings on a double harp is tuned diatonically — that is, in the manner of a major scale. The strings directly across from each other are tuned to the same pitch so that each string set is a mirror of each other. Essentially, this means that the harpist has two harps within a single harp frame capable of playing in the same range.
The fact double harps have two sets of strings instead of one is advantageous in that it allows harpists to create effects on the double harp that could not be realized on a single-course harp. For example, on a single-course harp, as is true for a pianist with the piano, the harpist can extend only so far in the instrument's range before the hands start to interfere with each other. With a double harp, this is not a problem, so the harpist can play two different items within the same range or even double a line. Additionally, if properly tuned, the strings on a double harp will vibrate sympathetically so that the sound of the harp is richer and more full.
Similar to other types of harps, double strung harps often utilize a lever system. Levers, when opened, raise the pitch of a harp string by one half step, allowing the harpist to move from flat to natural or natural to sharp, depending on the initial tuning of the harp. Subsequently, the harpist can switch from key to key and accommodate accidentals without manually adjusting the string by moving the peg to which it attaches. Combined with the fact double harps have two sets of strings, this means that, unlike other harps, double harps can play different notes of a pitch class within the same range simultaneously, such as C4 natural and C4 sharp.
Double harps come in all different sizes. Large harps have anywhere from 33 to 45 strings. The biggest ones are best suited for placement on the floor. Smaller double harps fall into the lap harp category. These are better for portability or for smaller harpists, as they are much lighter and don't take up as much space.
A double harp can use various types of strings, including nylon, gut and wire. In this way, it is no different than other harps. An advantage of double course harps, however, is that they can mix and match string types to achieve the best acoustic effect across the string sets. For example, if the manufacturer wanted to bring out the bass notes only on the left side, he could use wire strings on that section of the harp only, as wire strings tend to have a brighter sound that carries better. Therefore, the color possibilities available with a double strung harp are much greater than those for single-course harps.