Why does an "X" mark stand for a kiss?
In medieval times, most people were unable to read or write. When it came time to sign a document, people who could not write usually made an "X" mark. Of course, an "X" is not much of a signature. To add a sense of commitment, it became customary to kiss the "X" after writing it.
Kissing the "X" was "performance law," a ritual act that bound the parties the way legal documents bind us today. This act, witnessed by the person who wrote the text, represented a solemn guarantee of the truthfulness of what was written, and an oath to carry out whatever obligations were stated in the document.
Over the years, the "X" and the kiss became interchangeable. Today, people who can read and write might still add one or more "X" marks to their letters, maybe with a couple of "O"s thrown in for hugs.
Example: When I bought the house, I got the carpets and curtains thrown in. 我买房子的时候免费得到了地毯和窗帘。