One of the most investigated characteristics of speech production is its variability. The acoustic properties of a given speech sound (a consonant or vowel) and the associated movements of the vocal tract (the lips, jaw, tongue, velum, and larynx) exhibit variability from a number of sources. The factors that can influence the extent and degree of variation in articulation are extensive, ranging from the intent of the speaker's message to physical constraints imposed by the anatomy of the speech production mechanism. Identifying the specific factors is critical to understanding the nervous-system principles and processes underlying human communication.