Brugmann's law

Brugmann's law states that Proto-Indo-European *o became long *ā in open syllables in Proto-Indo-Iranian, but short *a in closed syllables.

Proto-Indo-Iranian: The reconstructed ancestor of the Indo-Aryan and Iranian language branches. Brugmann's law describes a vowel change during this ancestral language's development.
Karl Brugmann: A German linguist and leading figure in the Neogrammarian school of historical linguistics. Brugmann formulated the sound law that bears his name.
Sanskrit: An ancient Indo-Aryan language preserved in extensive religious, literary, and scholarly texts. Sanskrit supplies much of the early Indo-Aryan evidence for the law's vowel outcomes.
Grassmann's law: A dissimilation rule in Sanskrit and Greek that removes aspiration from one of two aspirated consonants in successive syllables. It is a separate Indo-European sound law, unlike Brugmann's syllable-conditioned vowel change.
Proto-Indo-European: The reconstructed ancestor of most languages in Europe and parts of Asia. The law tracks the fate of its short *o in Proto-Indo-Iranian.
Neogrammarians: A nineteenth-century school of linguists who argued that sound laws operate regularly. Their regularity principle shaped the debate over the law's exceptions and conditions.
Avestan: An ancient Iranian language in which the Zoroastrian scriptures are preserved. Avestan provides Iranian cognates for comparing inherited Indo-Iranian vowels.
Bartholomae's law: A Proto-Indo-Iranian sound law describing voicing and aspiration in clusters of stops and sibilants. It concerns consonant clusters rather than the vowel quantity central to Brugmann's law.
Open syllable: A syllable that ends in a vowel rather than a consonant. Proto-Indo-European *o lengthened when its syllable was open.
Indo-European linguistics: The comparative study of the languages descended from Proto-Indo-European. Brugmann's law arose from reconstructing sound correspondences across this language family.

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Osthoff's lawRelated: It is a contemporary named sound law from the same comparative-linguistic tradition.