Any musical instrument that produces sound by means of vibrating strings. The strings are under appropriate tension, and are set into vibration by being plucked (as the harp, guitar, or lute, etc.), strummed (as the harp, guitar, or lute, etc.), struck (as the piano, dulcimer, or clavichord, etc.), or bowed (as the violin, viol, or 'cello, etc.).