Personal union
 Personal unions can begin for very different reasons. The case that a princess who is already married to a king becomes pregnant, and their child inherits the crown of both countries is a rather common  cause. But a personal union sometimes was seen as a means against uprisings if a state wants an annexation of an other state. These unions can be written down in a constitution that clearly expresses that both states shall share the same person as head of state, but that is not always the case. Under these circumstances a personal union can easily be broken.