The Chi Rho ( ☧ , pronounced / ˈ k aɪ ˈ r oʊ / KY – ROH ; also called chrismon ) is one of the first symbols used to represent the name of Christ. It is created by combining the first two letters—chi (Χ) and rho (Ρ)—from the Greek word for "Christ," which is ΧΡ ΙΣΤΟΣ (rom: Christos). These letters are placed together so that the vertical line of the rho crosses the middle of the chi.

The Chi Rho symbol was used by Roman Emperor Constantine the Great (r. 306–337 AD) on a military flag called the Labarum. Earlier symbols similar to the Chi Rho included the Staurogram ( ) and the IX monogram ( ). When the Chi Rho is placed between the first and last letters of the Greek alphabet—alpha (Α) and omega (Ω)—it forms the Greek word Ᾰ̓́ΡΧⲰ (rom: archo), meaning "I rule."

Although the Chi Rho uses Greek letters, it is often used as a shortened form in Latin writing. In Latin texts, it may be written with endings that match Latin grammar, such as XPo, which means "to Christ," or χρ̅icola, meaning "Christian," as seen in the Latin song "Sumer is icumen in."

Origin and adoption

Before Christianity became widespread, the Chi-Rho symbol was used to mark important or valuable passages in the margins of pages. It was a short form of the Greek word "chrēston," meaning "good." Some coins from the time of Ptolemy III Euergetes, who ruled from 246 to 222 BC, were marked with the Chi-Rho symbol.

Lactantius, a Latin historian from North Africa who was helped out of poverty by Emperor Constantine the Great (who ruled from 306 to 337), wrote that Constantine dreamed of being told to place a "heavenly divine symbol" (in Latin, "coeleste signum dei") on the shields of his soldiers. Lactantius described the symbol Constantine chose the next day as unclear but similar to a Tau-Rho or a staurogram, both Christian symbols. On the same day, Constantine’s army fought Maxentius and won the Battle of the Milvian Bridge (312), which took place near Rome.

Eusebius of Caesarea, who died in 339, wrote about these events in two different ways. In his church history, written shortly after the battle, he did not mention a dream or vision. Instead, he compared Maxentius’s defeat (he drowned in the Tiber River) to the biblical story of Pharaoh’s defeat and said Constantine’s victory was due to divine protection.

Later, in a memoir about Constantine written after his death (around 337–339), Eusebius described a different version. He said that before the Battle of the Milvian Bridge, Constantine was thinking about how other leaders who asked many gods for help had failed. He decided to seek aid from the One God. At noon, Constantine saw a cross of light above the sun. The Greek words "Εν τούτῳ Νίκα!" ("In this, conquer!") appeared on the cross. Everyone in the army saw this miracle. That night, Christ appeared to Constantine in a dream and told him to create a sign based on what he had seen, which would protect him in battle.

Eusebius wrote that Constantine told him this story "and confirmed it with oaths" when they became close later in life. He added that if someone else had told the story, it would have been hard to believe.

Eusebius also described the labarum, a military flag used by Emperor Constantine in later battles against Licinius. This flag included the Chi-Rho symbol.

Later usage

An early image showing the link between Jesus' Crucifixion and his Resurrection appears on a 4th-century sarcophagus in Rome, called the sarcophagus of Domitilla. A wreath around the Chi-Rho symbol on this sarcophagus represents the idea that the Resurrection overcomes death.

After Emperor Constantine, the Chi-Rho became part of the official symbols used by the Roman Empire. Evidence shows that the Chi-Rho was placed on the helmets of some soldiers from the Late Roman period. Coins and medallions made during Constantine’s time also had the Chi-Rho. By 350, the Chi-Rho began appearing on Christian sarcophagi and wall paintings. Magnentius, a leader who claimed power, was the first to use the Chi-Rho with the letters Alpha and Omega on the back side of coins made in 353. In Roman Britain, a mosaic floor was found in Hinton St. Mary, Dorset, in 1963. Based on its style, it is from the 4th century. The center of the mosaic shows a beardless man wearing a robe and standing in front of the Chi-Rho, with pomegranates nearby, which symbolize eternal life. Another Chi-Rho was discovered in a fresco at a villa in Lullingstone. The symbol also appeared on Christian signet rings from the Late Roman period in Britain.

In 2020, archaeologists found a 5th-century chalice in Vindolanda, northern England, covered with religious images, including the Chi-Rho.

In Insular Gospel books, the start of Matthew 1:18—marking the beginning of Jesus’ life story—was often shown on a highly decorated page. The first word, "Christi," was abbreviated as "XPI" in Greek and often hidden by decorations. Although the letters "X" and "P" were not combined into a single symbol, these pages are called Chi-Rho pages. Famous examples include the Book of Kells and the Book of Lindisfarne. The "X" was seen as a symbol of the cross, a belief mentioned by writers like Isidore of Seville and other early Christian authors. The Book of Kells has another Chi-Rho abbreviation on page 124, which shows the Crucifixion. In some manuscripts, the Chi-Rho appears at the start of Matthew’s Gospel instead of at Matthew 1:18. In other works, such as the Carolingian Godescalc Evangelistary, the letters "XPS" in order, meaning "Christus," are shown in a prominent position.