Signing by Mark
A testator who is unable to create his or her own signature may execute the will by placing a mark on the document. Some reasons that a person cannot complete a signature may be illiteracy or a weakened physical condition. An "X" is commonly thought of as the type of mark associated with this type of execution.

The testator's mark alone is not sufficient to create a valid will, the mark must also be "subscribed" by another person. To subscribe a mark, someone must write the testator's name around the testator's mark after it has been made.

For Example:
The "X" is the only writing that has been made by the testator. All of the other writing has been done by the subscribing person on the testator's behalf. After the testator, Kurt Nilson, made an "X" on the document, the subscribing person wrote "Kurt Nilson" and "HIS MARK" around the "X" to show that it was made by Mr. Nilson.
      HIS
Kurt X Nilson
    MARK

Any will signed by a mark must have at least two subscribing witnesses who were present and observed the entire process. Without two subscribing witnesses, the will is improperly executed and cannot be accepted for probate.  Also, the person writing the testator's name around the mark cannot also sign as a subscribing witness.

See: Improper Execution, Generally; Signing By Another Person; will Validity, Generally