The Romans referred to Jupiter, but also to Jove. How did this work?
The nominative, Iuppiter, is derived from the archaic vocative Iū and pater (the word is a ‘syntactic compound’). The rest of the cases are derived from the stem Iov-, which A&G identify as related to the Greek Ζεύς through the PIE root *dyew. We can imagine that ‘Iovis‘ could serve a nominative, but the go-to nominative is definitely Iuppiter (or Iupiter).
(photo credit: Wiktionary)
You might be curious about why there’s a plural declension. Statues of Jove were also called ‘Joves.’
The Essential AG: 79, 79b, 100, 266c
Latin for Addicts 