You see this jotted in chalk on doorways all over Prague, and probably all over the Czech Republic and Slovakia as well.  At Christmastime, the chalk will be scrubbed off and a new inscription (K+M+B 2007) will be written to replace it.  The letters stand for the Three Wise Men, the Magi, of the Christmas story: Kaspar, Melchior, and Balthassar and it is thought to bring good luck in the coming year to commemorate them this way.