Why is the ninth month called September?
"duplicated, found in pairs," 1590s, from Latin geminatus "twinned, equal," past participle of geminare "to double, repeat," related to geminus "twin," perhaps from PIE *yem- "to pair." As a verb, from 1630s. Related: Geminated; geminating.
geminate gem·i·nate (jěm'ə-nĭt, -nāt')
Occurring in pairs.