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Old Indic diphthongs underwent a chain shift whereby e and o monophthongized (ai̯ > ē) and ai and au shortened, filling the gap (āi̯ > ai̯). While it is clear that these changes were complete by the end of the Vedic period, it has been unclear when during (or before) that period they occurred. Based on the avoidance of superheavy syllables in metrical cadences, which is corroborated here, I argue that both shifts took place during the compositional period of the R̥ gveda. That is, they had not yet begun when the oldest material was composed, but were largely complete by later material, including book 10.