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 ().
In pre-Christian times, the Chi-Rho symbol was also to mark a particularly valuable or relevant passage in the margin of a page, abbreviating chrēston (good). Some coins of Ptolemy III Euergetes (r. 246–222 BC) were marked with a Chi-Rho.
Although formed of Greek characters, the device (or its separate parts) is frequently found serving as an abbreviation in Latin text, with endings added appropriate to a Latin noun, thus XPo, signifying Christo, “to Christ”, the dative form of Christus.