Mich. Comp. Laws § 600.2144: Signature or handwriting; proof.

Sec. 2144.

Whenever in any suit or proceeding in any of the courts of this state, it shall be necessary or proper to prove the signature or the handwriting of any person, it shall be competent to introduce in evidence for the purpose of comparison, any specimen or specimens of the handwriting or signature of such person, admitted or proved to the satisfaction of the court to be genuine, whether or not the paper on which such handwriting or signature appears is one admissible in evidence or connected with the case or not. If such paper is not one admissible in evidence for some other purpose, or connected with the case, it shall not be admissible in evidence for the purpose of comparison unless it was made before the controversy arose concerning which such suit or proceeding was brought.