Brugmann's law

Brugmann's law, named for Karl Brugmann, is a controversial sound law stating that in the Indo-Iranian languages, an earlier Proto-Indo-European *o became *a in Proto-Indo-Iranian, but became *ā in open syllables when it was followed by a single 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.

The theory accounts for a number of otherwise puzzling facts. Sanskrit has pitaras, mātaras, bhrātaras for "fathers, mothers, brothers" but svasāras for "sisters", a fact neatly explained by the traditional reconstruction of the stems as *-ter- for "father, mother, brother" but *swesor- for "sister" (cf. Latin pater, māter, frāter but soror). Similarly, the great majority of n-stem nouns in Indic have a long stem-vowel, such as brāhmāṇas "Brahmins", śvānas "dogs" (from *ḱwones), correlating with information from other Indo-European languages that these were originally *on-stems. There is also an exception, ukṣan- "ox", which in the earliest Indic text, the Rigveda, shows forms as ukṣǎṇas "oxen". These were later replaced by "regular" formations (ukṣāṇas and so on, some as early as the Rigveda itself), but the notion that the short stem vowel might have been from an *en-stem is supported by the unique morphology of the Germanic forms, e.g. Old English oxa nom.singular "ox", exen plural—the Old English plural stem (e.g., the nominative) continuing Proto-Germanic *uhsiniz < *uhsenez, with e > i in noninitial syllables followed in Old English by umlaut. As in Indic, this is the only certain Old English n-stem that points to *en-vocalism rather than *on-vocalism.

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