The vocal tract is made up of the airway structures above the larynx that are used for speech production: the oral cavity, nasal cavities, velum, and pharynx.
Historical definitions of Cued Speech refer to the “natural mouth movements associated with speaking.” This presumes the the cuer speaks while cues. This was certainly true for hearing cuers that cued to their deaf infants. However, for deaf cuers, the mouth movements associated with cueing are not necessarily a byproduct of speaking.
Not all of the “natural mouth movements of speaking” are visible. Some sounds require that the velum prevent air from escaping through the nose (e.g., /p s b z/) and others require that air be directed through the nasal cavity (e.g. /m n/). This coordination is not available to the deaf lip reader or cue reader and must be learned if speech production is a goal. Controling the velum is not required to produce these segments while cueing silently.