Back when people sent a lot of letters and postcards through the mail, the position of a stamp could send a message of its own. The "code" became popular around 1890, after it was written up in a Hungarian newspaper. It was printed on postcards in several European languages so a correspondent could clue in a recipient for future reference. See a collection of these postcards explaining the meaning of your stamp at Poemas del río Wang. Link -via TYWKIWDBI