The first mention in literature of XXX for kisses at the bottom of a letter was in 1901, according to the Oxford English Dictionary. The X itself is very old. The custom goes back to the early Christian era, when a cross mark or "X" was the same as a sworn oath. The cross referred to the cross of Calvary and the first letter of the Greek word for Christ, Xristos. Even as little as a hundred and fifty years ago, not many people could read or write. The "X" at the bottom of a document took the place of a signature. They would kiss the "X" as a crucifix or bible was kissed to emphasize the importance of the mark. It was this practice that lead to the "X" representing a kiss. There isn't much known about the beginnings of the "O". It is a North American custom. The "O" represents the arms in a circle around another person. Arms crossed in front of you do not mean the "X" as a hug! The "O" is the hug. In "The Joys of Yiddish" by Leo Rosten, it is noted that illiterate immigrants (or those who did not know Roman-English letters) would generally sign entry forms with an "X" but Jews preferred an "O" to avoid making something that looked like a cross. Also, shopkeepers and salesmen would similarly sign receipts with a circle. Could this be the origin of the "O"? Or could it have developed from the game of tic tac toe, where one player uses the "X" and another the "O", leading people to think that the "O" was the hug, as the "X" is the kiss. http://www.hugkiss.com/valentine/xoxo.shtml

This tradition started with the Medieval practice of allowing those who could not write to sign documents with an "X". This was done before witnesses, and the signer placed a kiss upon the "X" to show sincerity. This is how the kiss came to be synonymous with the letter "X", and how the "X" came to be commonly used at the end of letters as kiss symbols. (Some believed "X" was chosen as a variation on the cross symbol, while others believe it might have been a pledge in the name of Christ, since the "X" -- or Chi symbol -- is the twenty-second letter of the Greek alphabet and has been used in church history to represent Christ.) Another thought is that it came from Jewish immigrants who would sign with an O instead of an X because they did not wish to mark their word with the obviously non-Jewish cross the X represented. Story of O is a French erotic novel. http://people.howstuffworks.com/valentine3.htm http://en.wikipedia.org/wiki/Hugs_and_Kisses

Because X is... supposedly like the shape your mouth makes puckered up and kissing, and O is the shape of your arms around someone.

The X represent two people kissing. Each side represents a person, like > and < Put them together and you get the symbol for kiss, X.