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According to legend, Emperor Constantine I saw this Christogram in the sky on the eve of the Battle of the Milvian Bridge. It is formed from χ (kh) and ρ (r), the first two letters of the Ancient Greek Χριστός (Khristós, “Christ”).

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1. The chi-rho, abbreviation of Christ.

• Often called chi-rho, or labarum, more rarely chrismon, crismon.