Chi-Rho is the oldest known monogram (or letter symbol) for Christ. Some call this symbol the "Christogram" and it dates back to the Roman Emperor Constantine (A.D. 306-337). Though the truth of this story is questionable, it is said that Constantine saw this symbol in the sky before an important battle, and he heard the message, "By this sign, conquer." Thus, he adopted the symbol for his army. Chi (x = ch) and Rho (p = r) are the first three letters of "Christ" or "Christos" in the Greek language. Though there are many variations of the Chi-Rho, most commonly it consists of the overlaying of the two letters, and oftentimes is surrounded by a circle.