Rex is an old Latin term not commonly used in modern English except in historical contexts or texts. The inscriptions referred to the ancient leader as 'rex'.
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Cultural Reference
'Rex' can evoke images of grandeur and power from Roman history when used in conversation or writing. In his role, he was as commanding as a rex.
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Not a Title
Unlike 'king', 'rex' is not used as a current formal title for a ruler. He rules like a rex, though he holds no crown.