Bonnell v. Bonnell
In Bonnell v. Bonnell, 117 Wis. 2d 241, 344 N.W.2d 123 (1984), the supreme court recognized that a spouse could transfer into the marital estate, by express or implied agreement or by gift, property that would otherwise be retained as the spouse's separate property. Bonnell, 117 Wis. 2d at 245.
The court held that the "transfer of separately owned property into joint tenancy changes the character of the ownership interest in the entire property into marital property which is subject to division." 117 Wis. 2d at 248.