The Chi-Rho emblem can be viewed as the first Christian Cross.

As a pre-Christian symbol, the Chi-ro signified good fortune. The Chi ro became an important Christian symbol when adopted by the Roman Emperor Constantine, representing the first two letters in the name of Christ- the Chi, or 'ch,' and Rho, or 'r.' According to Church Father Eusebius, on the eve of the Battle of the Milvan Bridge, the Emperor saw the emblem in a dream, with the inscription, "By this sign, you shall conquer." According to the story, the battle was won. In return for the victory, Constantine erected Christian churches. Unfortunately, this story is very unlikely, as Constantine's conversion occured on his deathbed, if at all. In any case, the symbol was the standard of the Emperor's army, prominently displayed on the Emperor's labarum, or battle standard.

Before it became the monogram of Christ, the chi rho was the monogram of Chronos, the god of time, and an emblem of several solar deities.

The Chi-ro is also the origin of the tradition of abbreviating "Christ" in "christian" or "Christmas" to "X."

In Hebrew, Chi-Rho equates to Tav-Resh. The chi rho was used in hermetic alchemical texts to denote time.