Chi Rho
The Chi Rho () is one of the earliest forms of christogram, and is used by some Christians. It is formed by superimposing the first two (capital) letters chi and rho (ΧΡ) of the Greek word "ΧΡΙΣΤΟΣ" = KRistos = Christ in such a way to produce the monogram. Although not technically a Christian cross, the Chi-Rho invokes the authority of Jesus, as well as symbolising his status as the Christ.
The Chi-Rho symbol was also used by pagan Greek scribes to mark, in the margin, a particularly valuable or relevant passage; the combined letters Chi and Rho standing for chrēston, meaning "good." Some coins of Ptolemy III Euergetes (r. 246–222 BC) were marked with a Chi-Rho.
The Chi-Rho symbol was used by the Roman emperor Constantine I as part of a military standard (vexillum), Constantine's standard was known as the Labarum. Early symbols similar to the Chi Rho were the Staurogram (
) and the IX monogram (
).
Christian accounts of Constantine's adoption of the Chi-Rho
According to Lactantius, a Latin historian of North African origins saved from poverty by the patronage of Emperor Constantine I (r. 306–337) as tutor to his son Crispus, Constantine had dreamt of being ordered to put a "heavenly divine symbol" (Latin: coeleste signum dei) on the shields of his soldiers. The description of the actual symbol chosen by Emperor Constantine the next morning, as reported by Lactantius, is not very clear: it closely resembles a Chi-Rho or a staurogram, a similar Christian symbol. That very day Constantine's army fought the forces of Maxentius and won the Battle of the Milvian Bridge (312), outside Rome.