Chi rho

The first two letters of Christ’s name in Greek, chi and rho, set one through the other. Lactantius writes that before his battle with Maxentius, Constantine was told in a dream to mark the heavenly sign of God on his soldiers’ shields, and marked them with the letter X with a line drawn through it, turned round at the top. Eusebius describes the standard that followed: a wreath of gold and precious stones, and within it “the letter P being intersected by X in its centre”. Here every point and opening of the monogram is found from the rim: one swing about its head, steps of the compass to its foot, the square round the rim for the chi, two halvings of the upper radius for the bowl. Only the ring outside is set by eye.