The Chi is the one that looks like an X. In the Greek alphabet it represents two letters, CH.
The Rho is the one that looks like a P. In the Greek alphabet it is R.
Together, the letters represent CHR, the first letters of the word CHRIST.
The Chi Rho is a Christogram - a combination of letters that forms an abbreviation for the name of Jesus Christ.
You will see Christograms in icons and in religious buildings, clothing and furnishings.
The Chi Rho is a common symbol and because it is compact, you will see it on quite small things like a chalice, a lapel pin or a pyx (a small box for communion wafers and a terrific Scrabble word).
Keep an eye out for this symbol next time you are in St Mary Magdalene’s.