Brugmann's law

In historical linguistics, Brugmann's law describes the outcomes of the Proto-Indo-European vowel *o in the Indo-Iranian languages based on the syllabic context in which the vowel occurred. In short, when the Proto-Indo-European short vowel *o occurred in an open syllable – that is, a syllable in which the vowel is the final element – it became the Proto-Indo-Iranian long vowel *ā so long as the syllable was not word-final. Otherwise, *o became the short vowel *a. The law is named for the German linguist Karl Brugmann.