Brugmann's law

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Brugmann's law, named for Karl Brugmann, is a sound law stating that in the Indo-Iranian languages, the earlier Proto-Indo-European *o normally became *a in Proto-Indo-Iranian but *ā in open syllables if it was followed by one consonant and another vowel. For example, the Proto-Indo-European noun for 'wood' was *dόru, which in Vedic became dāru. Everywhere else, the outcome was *a, the same as the reflexes of PIE *e and *a.