A personal union is the combination of two or more states that have the same monarch while their boundaries, laws, and interests remain distinct. A real union, by contrast, would involve the constituent states being to some extent interlinked, such as by sharing some limited governmental institutions. Unlike the personal union, in a federation and a unitary state, a central (federal) government spanning all member states exists, with the degree of self-governance distinguishing the two. The ruler in a personal union does not need to be a hereditary monarch.