The vocal folds or vocal cords are a pair of mucous membranes that stretch across the larynx. They can be vibrated during exhalation allowing a human being to form speech. During inhallation, they are relaxed and kept far apart, allowing air to enter. They are controlled by the vagus nerve.
Although they are not the only part of the upper respiratory tract involved in speech (the tongue, teeth and lips are also involved), they are responsible for a wide range in variation in frequency. The average frequency of the vocal folds establish the basic pitch of a person's voice. They tend to be larger in males, resulting in a lower pitch.