What is the Chi-Rho symbol?

Two letters, one name

In Greek, the language of the New Testament, Christ is Christos, ΧΡΙΣΤΟΣ in capitals. Its first letter, chi, looks like our X but sounds like the ch in Scottish loch. Its second, rho, looks like our P but is an R. Run the stem of the rho through the crossing of the chi and you have the monogram: ☧. So the “XP” on altar cloths and old gravestones is not X and P at all. It is “Chr.”

The same chi explains a word that annoys some people every December. In “Xmas,” the X is the Greek letter chi, standing for Christ, as it has in Christian abbreviations for centuries. It does not cross Christ out. The letters even had a life before Christianity: Greek scribes sometimes wrote the same ligature in a margin to flag a useful passage, short for chrēston, “good.” Christians gave it a new meaning, the name of their Lord (what “Christ” means).

The sign on Constantine’s shields

The Chi-Rho became famous in October 312, before Constantine’s battle with Maxentius at the Milvian Bridge (the whole story). Lactantius, a Christian teacher at his court, wrote a few years later that a dream told Constantine to mark the heavenly sign of God on his soldiers’ shields. His description is oddly worded: the letter X turned on its side, with its top bent round. Many scholars think that describes a slightly different monogram, the staurogram, a cross whose upright curls at the top into a rho. Christian scribes were already using it in Gospel manuscripts copied around AD 200, in the word for cross.

Either way, the Chi-Rho itself was soon on imperial objects. A silver medallion struck at Ticinum, today’s Pavia, around 315 shows Constantine in a helmet with a Chi-Rho badge.

The labarum

Eusebius, bishop of Caesarea, says he saw the emperor’s battle standard with his own eyes. It was a tall spear overlaid with gold, with a crossbar that made it a cross. At the top, a wreath of gold and precious stones held the two letters of Christ’s name, the rho crossed through its middle by the chi. From the crossbar hung a square banner rich with gold and jewels (Life of Constantine 1.31). The standard came to be called the labarum, a word nobody has convincingly explained. Around 327 a bronze coin of Constantinople showed it planted on a serpent, with the Latin legend SPES PVBLIC, the public hope.

Alpha and omega

From the fourth century the Chi-Rho often stands between two more Greek letters, alpha (Α) and omega (Ω), the first and last of the alphabet. They come from the book of Revelation:

I am the Alpha and the Omega, the First and the Last, the Beginning and the End.

The usurper Magnentius, who seized the West in 350, filled the back of his large bronze coins with a Chi-Rho between alpha and omega.

Where you can still see it

Across the late Roman world the sign marked anything Christian, and it has never gone out of use:

• Roman Britain. A fourth-century floor mosaic from Hinton St Mary, in Dorset, shows a man’s head in front of a Chi-Rho, often read as one of the earliest pictures of Christ. It is in the British Museum, with the Water Newton treasure, the oldest known set of Christian church silver from the empire, its plaques stamped with the sign.
• Rome. Fourth-century sarcophagi and catacomb gravestones carry it, sometimes inside a victor’s wreath.
• Today. It is embroidered on vestments and altar cloths and carved on church doors and tombs, and it even has its own character in the Unicode standard, ☧.