The vibration in the larynx is caused by an automatic modulation by the vocal cords of the air stream from the lungs. Analytically the mechanism is the same, and physically, closely analogous to that of the vacuum tube oscillator. It depends principally on the resonance of the vocal cords, the modulation of air friction in the glottis by their motion and the attraction due to constriction of the air stream between them. When these forces exist in certain relative proportions and phases, sustained oscillation as in singing takes place. The whole mechanism may be represented analytically by force equations, from which conditions for accretion or subsidence of the vibration or for sustained oscillation may be easily deduced. These equations also show the analogy with other types of oscillating systems.