The Chi Rho (/ˈkaɪ ˈroʊ/; also known as chrismon or sigla) is one of the earliest forms of christogram, formed by superimposing the first two (capital) letters—chi and rho (XP)—of the Greek word XPIΣTOΣ (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) 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.