PIE grammatical gender
As reflected in many daughters, PIE arbitrarily assigned all nouns to a concord class called “gender” which were reflected in other nominals. It is unclear at what point the feminine developed in PIE, but it is clear that by Proto-Core IE there were three grammatical genders: masculine, feminine, and neuter; and that early PIE had at least masculine and neuter.
Nouns all carried an intrinsic gender, whereas nearly all other nominals reflected the gender of the modified noun or antecedent. The nominals that did not reflect gender included a handful of quantifiers and pronouns in the first and second person. Hence, the majority of non-noun nominals included parallel declensions for each gender.