Page "Language family" Paragraph 4

Genealogically related languages present shared retentions, that is, features of the proto-language ( or reflexes of such features ) that cannot be explained by chance or borrowing ( convergence ).

Membership in a branch or group within a language family is established by shared innovations ; that is, common features of those languages that are not attested in the common ancestor of the entire family.

For example, what makes Germanic languages " Germanic " is that they share vocabulary and grammatical features that are not believed to have been present in Proto-Indo-European.

These features are believed to be innovations that took place in Proto-Germanic, a descendant of Proto-Indo-European that was the source of all Germanic languages.