And X is for......"Mr. X" or just "X." The designation often given to the unknown villain in detective novels. When the detective doesn't have enough information to guess much about the culprit in question, s/he often refers to them as "X." Or, conversely, if the detective thinks he knows who the villain is, but isn't yet ready to divulge the information. Philip MacDonald even went so far as to name one of his mysteries "Warrant for X."
X, of course, could also stand for "X marks the spot." Many classic crime novels employed the use of a map in the story. And often the site of the crime was marked with a nice tidy little X. Just so the reader would absolutely know where the foul deed was done. For those stories that involve a bit of a treasure hunt..."X marks the spot" takes on another meaning. Then it represents the place where the treasure seekers will find the hidden booty...whether that be pirate treasure in an adventure novel or the missing will in a mystery story. X can mean so many things...what does it mean for you?