As I noted before I blew out for the great old city of Philadelphia, bkmarcus is working on a strand about putting the X back in Xmas, and now (well, now in the vaguely outdated world of me, who has just returned to the ether from all too embodied world of the MLA) he has a post with words from Father Jim Tucker about the origins of the X. Good stuff therein.
All of which brought immediately to mind this beautiful XP (that's chi rho, but may be Greek to you as it is to me) page from the Book of Kells:
I remember gazing at a gazillion-times-enlarged projected slide of that page in college art history class. Seeing it little on the computer screen does not compare. Now I am thinking about how the ingenious illuminator combined the chi and rho in that image: they are not just superimposed as in some later combinations, but rather one and the same, different wigglings of the each other. It's a shame that rho is now invisible in Xmas, as if just because it can get absorbed in the chi it can be forgotten. Or maybe the rho prefers it that way, getting to come along for the ride without getting too much press.