Chi Rho

The Chi Rho is one of the earliest forms of christogram, and is used by Christians. It is formed by superimposing the first two (capital) letters chi and rho (ΧΡ) of the Greek word "ΧΡΙΣΤΟΣ" =Christ in such a way to produce the monogram. Although not technically a Christian cross, the Chi-Rho invokes the crucifixion of Jesus, as well as symbolizing 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 vexillum, which is known as Labarum. Early symbols similar to the Chi Rho were the Staurogram and the IX Monogram