The cross is also fairly easy to recognise as being alluded to.
It is often surrounded by a circle.
This post is the second in a new series reflecting on Christian signs, symbols, and similar.
Outside of the Christian context, Chi Rho was used to mark manuscripts and coins as χρηστόν (good, serviceable).
Eusebius, writing about Constantine’s success, claims that at noon before the battle of the Milvian Bridge in 312, Constantine saw a cross of light imposed over the sun. Attached to it, in Greek characters, was the saying “Τούτῳ Νίκα!” (in this sign conquer).
The “sign” is regularly understood to have been the Chi Rho.
The sun with rays is often seen in the image of the Ankh, which bears obvious similarities to the Chi Rho. There may be a connection between Constantine combining the God the Sun and God the Son – some would see echoes of this in the dating of Christmas.
The Ankh is often found in early Coptic art representing the Tree of Life. It is like a person with outstretched arms (orans). The Coptic Church accepted the ankh as a Christian symbol as early as the fourth century. In a number of murals and carvings from the 4th to the 9th century, the ankh is shown together with a Greek cross and a Chi Rho, showing the merging of meanings.