For as long as we can remember, 'X' has been the universal symbol for a kiss and 'O' has meant hugs.
But have you ever wondered why this letter became shorthand for a sign of affection?
Well, it turns out this little symbol actually has a very long history.
According to Marcel Danesi, author of The History of the Kiss: The Birth of Popular Culture, 'X' has religious meaning behind it.
"The X has always been a Christian symbol, and it is the first Greek-letter in the name of 'Christ,'" Danesi says.
During Medieval times, many people were illiterate, so when signing important documents, they would simply draw an 'X' as an abbreviation for the word 'Christ'. It was also customary for them to plant a physical kiss on the 'X'.
"As far as I can tell, official letters in the medieval period and even after were literally sealed with the 'X' - sealed with a kiss of faith, I guess."
"From this domain, the 'X' jumped into another domain, also to signify kissing but a different kind of kissing - romantic, rather than religious. At some point this became a symbolic practice among everyone," he says.
This reference to Christ can also explain how why Christmas is abbreviated to 'Xmas.'
So, why does 'O' mean hugs?
Said to be a North American invention, this symbol seems to have a relatively basic reason ...
Simply because X's and O's go together - like in tic-tac-toe or noughts and crosses!
You learn something new every day.