The Chi Rho (/ˈkaɪ ˈroʊ/; also known as chrismon) is one of the earliest forms of Christogram, formed by superimposing the first two (capital) letters—chi and rho (ΧΡ)—of the Greek word ΧΡΙΣΤΟΣ (Christos) in such a way that the vertical stroke of the rho intersects the center of the chi. The Chi-Rho symbol was used by the Roman Emperor Constantine I (r. 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.