Technically it’s a “[personal union](https://en.wikipedia.org/wiki/Personal_union)”. The same person is the head of state of more than one state, in this case fifteen states. In the same way, from 1603 England and Scotland were separate kingdoms ruled by the same monarch until they united to become the Kingdom of Great Britain in 1707. Ireland still remained a separate kingdom in personal union until it united to become part of the United Kingdom of Great Britain and Ireland in 1801.
The fifteen Commonweath realms are careful to keep their rules of succession identical to avoid ever having different monarchs. This was carefully orchestrated recently when the rules to give females an equal right of succession came into effect in 2015. Queen Victoria’s predecessor was also the King of Hanover (a German state) but Hanover did not permit females to rule, so that personal union came to an end when she ascended the British throne in 1837.
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