A sonometer is an apparatus by which the transverse vibrations of strings can be studied. It is also called the monocord because it often has only one string. On the box are two fixed bridges, near the ends, and at one end is a pulley. A string, often a steel wire, is fastened at one end, run over the bridges and the pulley, and attached to a weight holder hanging below the pulley. Weights can be added to the holder to produce tension in the wire, and a third, movable bridge, can be placed under it to change the length of the vibrating section of the string. A Sonometer demonstrates the relationship between the frequency of the sound produced by a plucked string, and the tension, length and mass per unit length of the string.