While I was researching the history of letter-writing today, I found an interesting article on "X-mark" signatures. These are sometimes found on old contracts and other documents, and mean that the person who made the "X" was unable to sign his or her own name because of illness or illiteracy. The clerk would then write "Her" (in the case of the signer being female) above the "X" and "Mark" below, then the first and last name of the signer on either side of the mark. There are a number of variations on the "X" -- the mark could look just like the upper-case letter, a cross, or even an upright line with three short marks across it.
Interestingly enough, a forthcoming project from Dromedary Press features an "X-mark" signature in its climactic chapter!