Answer
The staurogram is an ancient Christian symbol made of two Greek letters tau (Τ) and rho (Ρ), the first and last letters of the word "Christos" (Χριστός) in Greek. In the Middle Ages, it was also used to represent an angel. Phocas was an emperor of the Byzantine Empire who reigned from 602 to 610. During his reign, the staurogram was used on coins minted under his authority.