Before sexting, before coded bracelets, there were...stamps? Rio Wang has written a fascinating piece about how the angle that stamps were placed on the envelope conveys different meanings. According to the OP, the tradition began with the Austro-Hungarians in the 1860's and swiftly spread throughout Europe.
I highly recommend reading the original post, but here are some of the highlights:
Meanwhile, the English make it exceptionally formal, including the phrase: "Beware your dearest lady-friend!"
What's fascinating is that the code wasn't local to English speaking nations, there are guides in Russian, in Polish and in other languages I can't identify. Naturally, the French take romance to the next level:
Go read the full post! http://riowang.blogspot.com/2011/12/language-of-stamps.html