On today’s feast of the Epiphany it is a custom in many parts of the world for people to mark the front door of their house using specially blessed chalk. The date of the new year is inscribed as above, along with the letters C, M, and B, which stand for the three names traditionally given to the Magi – Caspar, Melchior, and Balthazar – or for Christus mansionem benedicat, ‘may Christ bless the house’. As well as the highly symbolic location of these letters, the use of chalk is also symbolic –  it is a substance from the earth itself, and while its traces will gradually fade away as the year passes, their meaning is invisibly inscribed forever in the hearts of the faithful.

There is a prayer for blessing chalk in the Rituale Romanum, one of the official Roman Catholic ritual books, but I don’t know how far back this tradition goes. Can anyone enlighten me?