The Origin of X's as Kisses at the End of Letters

        "The custom of writing X's for kisses in signing off a letter dates from the Middle Ages, when much of Europe's population remained illiterate.  On legal documents, in lieu of a signature, a person might write an X or the sign of the cross.  The signatory would then kiss the X, to promise to stick to the agreement.  Over later centuries, the written X came to mean not the signature but the kiss."

                   Language Visible:  Unraveling the Mystery of the Alphabet                         from A to Z, David Sacks, p. 243