bʰréh₂tēr

The kinship term *bʰréh₂tēr belongs to the same morphological class as *ph₂tḗr ("father") and *méh₂tēr ("mother"), characterised by the agent-noun suffix *-tēr. The initial consonant cluster *bʰr- undergoes regular treatment in each branch: Latin f- (from *bʰ- by Italic sound change), Greek phr- (from *bʰr- with aspiration shift), Germanic b- (from *bʰ- with Grimm's Law devoicing not applying to voiced aspirates in the same way as plain voiced stops), and Sanskrit bhr-. Latin frāter ("brother") preserves the kinship meaning directly. Its derivatives—fraternal, fraternity, fratricide—entered English through learned borrowing. The long vowel ā reflects the regular Latin outcome of *eh₂. Greek phrā́tēr (φράτηρ) underwent a notable semantic shift: rather than denoting a biological brother, it came to mean "member of a phratry" (φρατρία), a kinship-based social division in the Greek city-state. The biological sense was taken over by adelphós (ἀδελφός, literally "of the same womb"). This semantic replacement provides evidence for the social reorganisation of kinship terminology in early Greek society. Sanskrit bhrā́tā (nominative singular) continues the root with full regularity. The Avestan cognate brātar- confirms the Indo-Iranian reconstruction. In Germanic, Old English brōþor (Modern English brother), Old High German bruoder (Modern German Bruder), Old Norse bróðir, and Gothic broþar all continue Proto-Germanic *brōþēr. The consonant cluster *-þ- (rather than expected *-t-) in Germanic has been the subject of discussion, with Verner's Law invoked to explain the voicing. Old Irish bráthair, Welsh brawd, Lithuanian brólis (with a diminutive suffix replacing the original *-tēr), and Old Church Slavonic bratŭ complete the picture. The near-universal retention of this term across all branches, like the parental kinship terms, testifies to the stability of core family vocabulary in Indo-European.