The Chi Rho is one of the earliest christograms used by Christians. It is formed by superimposing the first two letters in the Greek spelling of the word Christ ( Greek : "Χριστός" ), chi = ch and rho = r, in such a way to produce the monogram . 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."

Although not technically a cross, the Chi Rho invokes the crucifixion of Jesus as well as symbolizing his status as the Christ. There is early evidence of the Chi Rho symbol on Christian Rings of the third century.

The labarum (Greek: λάβαρον) was a vexillum (military standard) that displayed the "Chi-Rho" symbol, formed from the first two Greek letters of the word "Christ" (Greek: ΧΡΙΣΤΟΣ, or Χριστός) — Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine I. Since the vexillum consisted of a flag suspended from the crossbar of a cross, it was ideally suited to symbolize crucifixion. The Chi-Rho symbol was also used by Greek scribes to mark, in the margin, a particularly valuable or relevant passage; the combined letters Chi and Rho standing for chrēston, meaning "good."