One of the oldest Christograms is the Chi-Rho. It consists of the superimposed Greek letters chi(Χ) and rho(Ρ), which are the first two letters of Greek χριστός "Christ". It was displayed on the labarum military standard used by Constantine I in AD 312. The IX monogram () is a similar form, using the initials of the name Ἰησοῦς (ὁ) Χριστός "Jesus (the) Christ", as is the ΙΗ monogram (), using the first two letters of the name Ἰησοῦς "Jesus".
There were a very considerable number of variants of "Christograms" or monograms of Christ in use during the medieval period, with the boundary between specific monograms and mere scribal abbreviations somewhat fluid.
The name Jesus, spelt "ΙΗΣΟΥΣ" in Greek capitals, has the abbreviations IHS (also written JHS, IHC, or ΙΗΣ), the name Christus , spelt "ΧΡΙΣΤΟΣ", has XP (and inflectional variants such as IX, XPO, XPS, XPI, XPO, XPM). In Eastern Christian tradition, the monogram ΙϹΧϹ (with Overline indicating scribal abbreviation) is used for Ἰησοῦς Χριστός in both Greek and Cyrillic tradition.
A Middle Latin term for abbreviations of the name of Christ is chrisimus.Similarly, Middle Latin crismon, chrismon refers to the Chi Rho monogram specifically.
In Plato's Timaeus, it is explained that the two bands which form the "world soul" (anima mundi) cross each other like the letter chi, possibly referring to the ecliptic crossing the celestial equator.Justin Martyr in the 2nd century makes explicit reference to Plato's image in Timaeus in terms of a prefiguration of the Holy Cross. and an early testimony may be the phrase in Didache, "sign of extension in heaven" (sēmeion epektaseōs en ouranōi).
An alternate explanation of the intersecting celestial symbol has been advanced by George Latura, claiming that Plato's visible god in Timaeus is in fact the intersection of the Milky Way and the Zodiacal Light, a rare apparition important to pagan beliefs that Christian bishops reinvented as a Christian symbol.
The most commonly encountered Christogram in English-speaking countries in modern times is the Χ (or more accurately, the Greek letter chi), representing the first letter of the word Christ, in such abbreviations as Xmas (for "Christmas") and Xianor Xtian (for "Christian").