Why is the ninth month called September?
late 14c., "alphabetical arrangement of all the words in a book" (especially the Bible), from Old French concordance (12c.) "agreement, harmony," from Late Latin concordantia, from concordantem (nominative concordans; see concord). Originally a citation of parallel passages. Literal meaning "fact of agreeing" attested in English from mid-15c.
concordance con·cor·dance (kən-kôr'dns)
The presence of a given trait in both members of a pair of twins.