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Western Rome

The Chi-Rho symbol was used by the Roman Emperor Constantine I (regnum. 306–337 AD) 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.

The Chi Rho, also known as chrismon, is one of the earliest forms of christogram, formed by superimposing the first two (capital) letters -chi and -rho of the Greek word (Christos) in such a way that the vertical stroke of the rho intersects the center of the chi.