"The vocal cords (vocal folds) are paired bands of fibrous tissue near the base of the larynx.  In normal breathing there is a V-shaped gap between them, called the glottis.  Sound is produced when the cords close together, tighten by muscle action, and vibrate as air from the lungs passes between them.  The greater the tension in the cords, the higher the pitch (frequency).  Above are the false vocal cords (vestibular folds).  These do not produce sound but help close off the larynx when swallowing."—Thank you, Steve Parker, for The Human Body Book, Dorling Kindersley Limited, 2007, p.137