A string is the vibrating element that produces sound in string instruments such as the guitar, harp, piano, and members of the violin family. Strings are lengths of a flexible material that a musical instrument holds under tension so that they can vibrate freely, but controllably. Strings may be "plain" (consisting only of a single material, like steel, nylon, or gut). "Wound" strings have a "core" of one material, with an overwinding of other materials. This is to make the string vibrate at the desired pitch, while maintaining a low profile and sufficient flexibility for playability.