Where did the custom of putting Xs and Os at the bottom of a letter come from?

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"? 就跟阿Q画圈一样啊！