There is something about an X that goes a long way back and we keep returning to it over and over again to create key symbols in our visual iconography. The common 'X' opens out some very interesting features about the relationship between drawing and language. I have touched on some of these issues before, often when looking at those entoptic forms that humans have been using for thousands of years such as spots, dots and zig-zags.
Entoptic categories, each represented here by a typical form
We have all signed off a text message with XX to mean kisses, which is a custom in language that dates back to the Middle Ages, when a Christian cross was drawn on documents or letters to mean sincerity, faith or honesty. It was also used in early Christian history to stand for 'Christ' which in Greek was χριστός the X standing in for the entire word. This was of course quite a useful thing to do when Christianity was a forbidden religion. The issue here is how the kiss, a physical act between two people, becomes graphically represented by an X. It does look a little like two faces coming together, and more importantly that tiny point of contact where we would expect the mouths to be is in the X a visually and psychologically heightened moment.