in the throat and not visible to them. If an abductory movement or adductory movement is strong enough, the vibrations of the vocal folds will stop (or not start). If the gesture is abductory and is part of a speech sound, the sound will be called voiceless. However, voiceless speech sounds are sometimes better identified as containing an abductory gesture, even if the gesture was not strong enough to stop the vocal folds from vibrating. This anomalous feature of voiceless speech sounds is better understood if it is realized that it is the change in the spectral qualities of the voice as abduction proceeds that is the primary acoustic attribute that the listener attends to when identifying a voiceless speech sound, and not simply the presence or absence of voice (periodic energy). An adductory gesture is also identified by the change in voice spectral energy it produces. Thus, a speech sound having an adductory gesture may be referred to as a "glottal stop" even if the vocal fold vibrations do not entirely stop. Other aspects of the voice, such as variations in the regularity of vibration, are also used for communication, and are important for the trained voice user to master, but are more rarely used in the formal phonetic code of a spoken language. Physiology and vocal timbre The sound of each individual's voice is entirely unique not only because of the actual shape and size of an individual's vocal cords but also due to the size and shape of the rest of that person's body, especially the vocal tract, and the manner in which the speech sounds are habitually formed and articulated. (It is this latter aspect of the sound of the voice that can be mimicked by skilled performers.) Humans have vocal folds that can loosen, tighten, or change their thickness, and over which breath can be transferred at varying pressures. The shape of chest and neck, the position of the tongue, and the tightness of otherwise unrelated muscles can be altered. Any one of these actions results in a change in pitch, volume, timbre, or tone of the sound produced. Sound also resonates within different parts of the body, and an individual's size and bone structure can affect somewhat the sound produced by an individual. Singers can also learn to project sound in certain ways so that it resonates better within their vocal tract. This is