The mark at the centre of the collection is the Chi-Rho. Chi (Χ) and Rho (Ρ) are the first two letters of Christos, the Greek word for Christ, joined into one sign small enough to cut into stone in a few strokes.
Before the Battle of the Milvian Bridge in 312, Constantine had his soldiers mark their shields with the sign of Christ. Within a generation the Chi-Rho was on coins, on sarcophagi and on the walls of the catacombs.
How we draw it
On the Christogram artwork it sits inside an oval of rays at the centre of a Gothic façade, the way a window or a reliquary would hold it.
Alpha and Omega beside it
The first and last letters of the Greek alphabet stand either side of the Chi-Rho, as they did on early tombs. It is also why our first collection is called Alpha.