Modern representations of the Chi-Rho sign represent the two lines crossing at ninety degree angles, the early examples of the Chi-Rho cross at an angle that is more vividly representative of the chi formed by the solar ecliptic path and the celestial equator. This image is most familiar in Plato's Timaeus, where it is explained that the two bands which form the "world soul" (anima mundi) cross each other like the letter chi. Not only did the two legs of the chi remind early Christians of the Holy Cross, "it reminded them of the mystery of the pre-existent Christ, the Logos Theou, the Word of God, who extended himself through all things in order to establish peace and harmony in the universe," in Robert Grigg's words. Hugo Rahner summarized the significance: "The two great circles of the heavens, the equator and the ecliptic, which, by intersecting each other form a sort of recumbent chi and about which the whole dome of the starry heavens swings in a wondrous rhythm, became for the Christian eye a heavenly cross." Of Plato's image in Timaeus, Justin Martyr, the Christian apologist writing in the 2nd century, found a prefiguration of the Holy Cross,[#] and an early testimony may be the phrase in Didache, "sign of extension in heaven" (sēmeion ekpetaseōsen ouranō).
After Constantine, the Chi-Rho became part of the official imperial insignia. Archaeologists have uncovered evidence demonstrating that the Chi-Rho was emblazoned on the helmets of some Late Roman soldiers. Coins and medallions minted during Emperor Constantine's reign also bore the Chi-Rho. By the year 350, the Chi-Rho began to be used on Christian sarcophagi and frescoes. The usurper Magnentius appears to have been the first to use the Chi-Rho monogram flanked by Alpha and Omega, on the reverse of some coins minted in 353. In Roman Britannia, a tesselated mosaic pavement was uncovered at Hinton St. Mary, Dorset, in 1963. On stylistic grounds, it is dated to the 4th century; its central roundel represents a beardless male head and bust draped in a pallium in front of the Chi-Rho symbol, flanked by pomegranates, symbols of eternal life. Another Romano-British Chi-Rho, in fresco, was found at the site of a villa at Lullingstone (illustrated). The symbol was also found on Late Roman Christian signet rings in Britain.