In humans, vocal cords, also known as vocal folds or voice reeds, are folds of tissue in the throat that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for speech or singing, the folds are controlled via the recurrent laryngeal branch of the vagus nerve. They are composed of twin infoldings of mucous membrane stretched horizontally, from back to front, across the larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation.

The 'true vocal cords' are distinguished from the 'false vocal folds', known as vestibular folds or ventricular folds, which sit slightly superior to the more delicate true folds. These have a minimal role in normal phonation, but can produce deep sonorous tones, screams and growls.

The length of the vocal fold at birth is approximately six to eight millimeters and grows to its adult length of eight to Sixteen millimeters by adolescence. Testosterone, an androgen secreted by the gonads, causes irreversible changes in the cartilages and musculature of the larynx when present in high enough concentrations, such as during an adolescent boy's puberty: The thyroid prominence appears, the vocal folds lengthen and become rounded, and the epithelium thickens with the formation of three distinct layers in the lamina propria.

 

Note:   The above text is excerpted from the Wikipedia article Vocal cords, which has been released under the GNU Free Documentation License.