Both Sanskrit and Ancient Greek reflexes have a radical stress throughout the paradigm, which indicates that this accentuation was original, and that the PIE word for “brother” had an acrostatic inflection. Archaicity of the paradigm is furthermore indicated by the fact that the rest of the inherited kinship terms in Sanskrit have suffixal and desinential accent, so the pattern for the word for “brother” could not arise by analogy. The hysterokinetic inflection of other kinship terms (e.g. for *ph₂tḗr (“father”) and *dʰugh₂tḗr (“daughter”), both of which end in *-tḗr) did exert influence in the two, which is evident by the absence of zero-grade in the suffix syllable (Sanskrit forms with -ता (-tā), -तरम् (-taram) etc., Greek with -τηρ (-tēr), -τερος (-teros) etc.).
The root vowel is short in all of the reflexes, and no trace of lengthened grade can be found: in no language did Eichner's law operate (which predicts non-coloration of the *ē in **bʰrḗh₂-tr). Furthermore, in Balto-Slavic one can find an acute vowel (reflecting *bʰréh₂) instead of a circumflex vowel (which would reflect *bʰrḗh₂).
This lexeme is widespread, though absent from Albanian and rare in Anatolian. Because it means “kinsman” in Greek, and meant both “brother” and “kinsman, cousin” (or “comrade”) in Celtic (e.g. Old Irish bráthair) and Baltic (e.g. in Latvian and Old Prussian), some suspect it had similarly wider meaning in PIE (even the English brother is often used to refer to socially-affiliated non-relatives).
- Lydian: 𐤡𐤭𐤠𐤱𐤭𐤳𐤦𐤳 (brafrsis) (brafrsis; stem brafr-)
- Proto-Balto-Slavic: *brā́ˀtē (see there for further descendants)
- Proto-Celtic: *brātīr (see there for further descendants)
- Proto-Germanic: *brōþēr (see there for further descendants)
- Proto-Hellenic: *pʰrā́tēr (see there for further descendants)
- Proto-Indo-Iranian: *bʰráHtā (see there for further descendants)
- Proto-Italic: *frātēr (see there for further descendants)
- →? Doric Greek: βρά (brá)
- Mysian: βρατεραις (braterais, dat.pl.)
- Phrygian: βρατερε (bratere, dat.sg.)
- Proto-Tocharian: *procer (see there for further descendants)