Rengao, a language of Vietnam exhibits phonological effects characteristic of Mon-Khmer ‘register’ languages generally. It is claimed that the basis of register is tongue-root position (pharynx width). After a brief discussion of pharyngeal effects in the phonology, those effects are further traced in the sound symbolism of Rengao expressives (ideophones). A magnitude symbolism principle is identified which is expressed as a diminutive-augmentative opposition involving both consonants and vowels. Though oral cavity symbolism has been broadly documented. Rengao demonstrates the importance of pharyngeal cavity symbolism also. The two cavity effects are shown to interlock forming one integrated symbolic system.