Wis. Stat. § 700.20: Extent of undivided interests in tenancy in common.

The extent of the undivided interests of tenants in common for the duration of the tenancy is determined by the intent expressed in the document of title, instrument of transfer or bill of sale; if no intent is expressed in the document, instrument or bill of sale, tenants in common are presumed to own equal undivided interests for the duration of the tenancy.