A string instrument (or stringed instrument) is a musical instrument that produces sound by means of vibrating strings. In the Hornbostel-Sachs scheme of musical instrument classification, used in organology, they are called chordophones. The most common string instruments in Western music are those in the violin, piano and guitar families.

Plucking
Instruments such as the guitar, oud, sitar, banjo and harp are plucked, either by a finger or thumb, or by some type of plectrum. This category includes the keyboard instrument the harpsichord, which formerly used feather quills (now plastic plectra) to pluck the strings.

Bowing
Bowed string instruments include the violin, viola, cello (of the violin family) and the double bass (of the viol family). The bow consists of a stick with many hairs stretched between its ends. Bowing the instrument's string causes a stick-slip phenomenon to occur, which makes the string vibrate.
Other bowed instruments are the hardingfele, nyckelharpa, kokyū, rebec, erhu, igil, kamanche, and sarangi. The hurdy gurdy is bowed by a wheel.

Striking
The third common method of sound production in stringed instruments is to strike the string with a hammer. By far the most well-known instrument to use this method is the piano, where the hammers are controlled by a mechanical action; another example is the hammered dulcimer, where the player holds the hammers.
A variant of the hammering method is found in the clavichord: a brass tangent touches the string and presses it to a hard surface, inducing vibration. This method of sound production yields a soft sound. The maneuver can also be executed with a finger on plucked and bowed instruments; guitarists refer to this technique as "hammering-on". Violon-family string instruments are also occasionally instructed to strike the string with the side of the bow, a technique called col legno. This yields a percussive sound along with the pitch of the note. A well-known use of col legno for orchestral strings is the Gustav Holst's "Mars" movement from The Planets suite.

Other methods
The aeolian harp employs a very unusual method of sound production: the strings are excited by the movement of the air.
Some string instruments have keyboards attached which are manipulated by the player, meaning they do not have to pay attention to the strings directly. The most familiar example is the piano, where the keys control the felt hammers by means of a complex mechanical action. 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 tiny plectra).
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 to "bow" them with bow hair wrapped around the strings, or play them by rolling the bell of a brass instrument such as a trombone on the array of strings.
Other keyed string instruments, small enough to be held by a strolling player, include the plucked autoharp, the bowed nyckelharpa, and the hurdy gurdy, which is played by cranking a rosined wheel.
Steel-stringed instruments can be played using a magnetic field. An E-Bow is small hand-held battery-powered device which can be used to excite the strings of an electric guitar. It provides a sustained, singing tone on the string which is magnetically-vibrated.