A string instrument is a musical instrument that produces sound via vibrating strings. They may also be called chordophones, a name deriving from the Hornbostel-Sachs scheme of musical instrument classification.
In order for a string instrument to produce sound, its string or strings must vibrate. There are two common ways of bringing this about: instruments such as the guitar and kora are plucked, either by a finger or thumb, or by some other device such as a plectrum; while instruments like the cello and rebec are usually played by drawing a bow across the strings. However, instruments normally bowed are occasionally plucked (this is known as pizzicato), and instruments normally plucked are sometimes played with a bow (Jimmy Page of Led Zeppelin sometimes played the electric guitar this way, for example).
Instruments which are normally bowed such as the violin, etc, must be bowed perpendicularly to the string, usually at a point half way between the end of the fingerboard and the bridge. Bowing very the near the bridge (known as sul ponticello) or above the fingerboard (sul tasto) are special techniques which produce a different sound.
Less commonly, the strings are agitated by other means. The player of the hammered dulcimer strikes the strings with hand-held hammers, while the strings of the aeolian harp are excited by the movement of the air.
Some string instruments have keyboards attached which are manipulated by the player, meaning he does not have to pay attention to the strings directly. Perhaps the most familiar example is the piano, where felt hammers are made to strike the strings inside the instrument when the keys are depressed (it should be noted, however, that the piano is often considered a percussion instrument, due to the fact that the strings are struck). Other string instruments with a keyboard include the clavichord (where the strings are struck by tangents), and the harpsichord (where the strings are plucked by quills).
With these keyboard instruments too, the strings are occasionally plucked or bowed by hand. Composers such as Henry Cowell wrote music which asks for the player to reach inside the piano and pluck the strings directly, or wrap bow-hair around the strings and play them with that.
A vibrating string on its own makes only a very quiet sound, so string instruments are usually constructed in such a way so as this sound in amplified either by a hollow chamber, a solid surface, or both. On the violin, for example, the taut strings pass over a bridge resting on a hollow box. The strings' vibrations are distributed via the bridge to all surfaces of the instrument, and thus amplified.
A single string of a constant tension will only produce one note, so to obtain further notes string instruments employ two methods. Most instruments have more than one string - in the case of the harp or piano, for example, this is the only way in which extra notes are obtained. With instruments such as the violin or guitar the player may press down on the strings with their fingers or some other device in order to effectively shorten the length of it which vibrates. This is known as stopping the string. In such instruments, a fingerboard is often attached to the resonating box - it is between this and the player's finger that the string is stopped.
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