Brugmann's Law states that Proto-Indo-European *o in open syllables reflects as Indo-Iranian long a (ā).
This appears not to apply when the *o in question does not ablaut (*poti- > Sanskrit pati-), and doesn't apply when the *o is in a closed syllable (*kowHi- > Skt kavi-); in such cases, the regular outcome in InIr is short a.
*swesorm= 'sister' > Skt svasāram
*kwetwores 'four' > Skt catvāras
Named after the Indo-Europeanist Karl Brugmann.