Wis. Stat. § 889.23

Every written instrument, except promissory notes and bills of exchange, and wills, may be proved or acknowledged in the manner now provided by law for taking the proof or acknowledgment of conveyances of real estate and when so proved and acknowledged shall be competent evidence whenever it is relevant. Any instrument, which is attested but which is not required by law to be witnessed, may be proved as though there were no subscribing witness thereto.