Signing By Mark
A principal who is unable to create his or her own signature may execute a power of attorney 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 this type of signature.
The principal's mark alone is not typically sufficient to create a valid document, the mark must also be "subscribed" by another person. To subscribe a mark, another person must write the principal's name around the principal's mark.
The "X" is the only writing that has been made by the principal. All of the other writing has been done by the subscribing person on the principal's behalf. After the principal, John Smith, made an "X" on the document, the subscribing person wrote "John Smith" and "HIS MARK" around the "X" to show that it was made by Mr. Smith.
John X Smith
Every power of attorney signed by a mark must have at least two subscribing witnesses who were present and observed the entire process. Without two subscribing witnesses, the document is improperly executed and cannot be followed.
See: Signing By Another Person; Witnesses, Generally