Personal union
Personal unions can begin for very different reasons.  In many cases a princess who is already married to a king becomes pregnant, and their child inherits the crown of both countries.  However, a personal union sometimes was seen as a means against uprisings if a state wants an annexation of an other state.  The unions can be written down in a constitution, which clearly expresses that both states shall share the same person as head of state.  When that is not the case, a personal union can easily be broken.