"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 ().
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." - (en.wikipedia.org 20.12.2021)
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