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