A vibrating string can produce a motion that is rich in harmonics (different frequencies of vibration). Bowing the string not only allows a range of expressive techniques, but also supplies energy continuously and so maintains the harmonic richness. However, a string on its own makes little sound (think of an electric guitar that's not plugged into an amplifer). The bridge and body of the violin, and other related instruments, serve to transmit some of the vibrational energy of the string into the air as sound. The way in which they do so is important to the sound of the violin family of instruments.