History Facts The letter “X” has ambiguous linguistic roots, and has carried various meanings in different contexts. “X” marks the spot of buried treasure on a map, represents a variable in mathematics, or can be used to select a choice on a ballot. Nowadays, it’s also commonly used as a symbol of affection and endearment — especially when it’s paired with “O” to form “XO,” signifying kisses and hugs. But how exactly did “X” come to represent a kiss, and when did that originate?
The origins of “X” being associated with a kiss can be traced back to the Middle Ages. In an era when literacy rates were low and formal education was a rare privilege, people who couldn’t write would sign documents with an “X” instead of their name. When people signed with an “X,” it wasn’t merely a mark; it was a symbol that carried the weight of an oath. To validate their intentions and their “signature,” people were also known to kiss the “X.”
How the letter “X” transitioned from a kiss in the name of sincerity to a kiss of romance or affection isn’t clear. The Oxford English Dictionary traces the earliest known use of that meaning to a 1763 letter by British naturalist Gilbert White. In the letter, White signs off, “I am, with many a xxxxxxx and many a Pater noster and Ave Maria, Gil White.” This interpretation, however, has been challenged: Stephen Goranson, a researcher at Duke University, instead suggests that the “X” likely represented blessings, not kisses, given its use alongside religious phrases such as “Ave Maria.” Indeed, an “X” was historically used as a symbol of the Christian cross. “X” is also the first letter in the Greek word for Christ, Χριστός — hence the well-known abbreviation for Christmas, “Xmas.”