[Verginius Rufus]

Petra is Greek for "rock." The Latin word appears to be simply a loan-word from Greek.

St. Paul calls St. Peter "Kephas" (accent on the last syllable) in Galatians 1.18, 2.9, and 2.11 (but "Petros" in Gal. 2.7 & 2.8).

[Mrs. Don-o]

Thank you. I learn something new every day!