It is a development from personal union and has historically been limited to monarchies.
Unlike personal unions, real unions almost exclusively led to a reduction of sovereignty for the politically weaker constituent.
That was the case with Lithuania and Norway, which came under the influence of stronger neighbors, Poland and Denmark respectively, with which each of them had shared a personal union previously.
The most notable example of such a move is the Kingdom of Hungary (Lands of the Crown of Saint Stephen), which achieved equal status to Austria (which exercised control over the "Cisleithanian" crown lands) in Austria-Hungary following the Austro-Hungarian Compromise of 1867.
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