A string is the vibrating element that is the source of vibration 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 freely vibrate. Strings may be "plain" (consisting only of a single material, like steel, nylon, or gut) or they may have a core of one material with an overwinding of other materials to add mass so the string sounds the desired pitch, while keeping the string thin and flexible enough to play. Such strings are called "wound strings."