A personal union is a relationship of two or more sovereign states, which, through law, share the same person as their head of state.
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.
Personal union with the Congo Free State from 1885 to 1908, when it annexed the Congo Free State.
Personal union with Belgium from 1885 to 1908, when Belgium annexed it.
The actual situation was slightly more complex with the Dutch provinces Holland, Zeeland, Utrecht, Gelderland and Overijssel entering into personal union in 1689 and Drenthe in 1696. Only 2 Dutch provinces never entered into the personal union: Friesland and Groningen.