Chi-Rho Symbol
The Chi-Rho symbol is an anagram of the first two letters of Christ in Greek, the Chi (our CH) and the Rho (our R). These two letters written together stand for Christianity in the Late Roman and Early Medieval periods.
(from Cluny Museum in Paris)
In case you were wondering, the reason some chi-rho's have a Greek letter alpha (A) on one side of the chi-rho and a Greek letter omega (looks like a W) on the other side (this carving is missing the alpha) is that the Bible says that Jesus said "I am the alpha and the omega, the beginning and the end" (because omega was the last letter of the Greek alphabet, the way Z is our last letter in English).
