Chru, a Chamic language of south-central Vietnam, has been described as combining contrastive obstruent voicing with incipient registral properties (Fuller, 1977). A production study reveals that obstruent voicing has already become optional and that the voicing contrast has been transphonologized into a register contrast based primarily on vowel height (F1). An identification study shows that perception roughly matches production in that F1 is the main perceptual cue associated with the contrast. Structured variation in production suggests a sound change still in progress: While younger speakers largely rely on vowel height to produce the register contrast, older male speakers maintain a variety of secondary properties, including optional closure voicing. Our results shed light on the initial stages of register formation and challenge the claim that register languages must go through a stage in which breathiness or aspiration is the primary contrastive property (

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