Real union

Real union is a union of two or more states, which share some state institutions in contrast to personal unions; however, they are not as unified as states in a political union. 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, Scotland and Norway which came under the influence of stronger neighbors, Poland, England and Denmark respectively, with whom each of them had shared a personal union previously. Sometimes, however, a real union came about after a period of political union. 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.
Finland under Russian rule.
The Russian Empire never recognized Finland as a state in its own right. However, Finland was, for most of this time, governed in accordance with its semi-official constitution, which eventually became a norm leading the bulk of the Finnish elite (and later, of the common people as well) to consider Finland a separate constitutional monarchy in real union with Russia.
Before 1910, there was no codification of the relationship between Finland and Russia in Russian law, so the Finnish Senate and the Finnish Diet in particular were, for a long period of time, spanning from the revival of the Diet of the Estates in 1863 to the threshold of russification in the 1880s, allowed to act as if Finland was a state separate from that of Russia. As a result, the eventual subjugation of the Finnish Diet to Russian governmental organs in between 1899 and 1905 and again from 1908 was viewed as unconstitutional in Finland, as was the 1910 codification of Finland's position as an indivisible part of the Russian Empire governed according to a "set of special laws".
After the February Revolution in March 1917, the Russian Provisional Government repealed all of the restrictions imposed on Finnish autonomy from February 1899. In 1917, Finnish politicians from the right to the left sought to have the new Russia recognise Finland as a separate state with its own constitution. The Social Democrats passed the "Power Act" in July 1917, which effectively meant real union, but the Provisional Government, after having survived a Bolshevik coup attempt the same month, dissolved the parliament.
In the autumn of 1917, a committee was set up in Finland to draw up a proposal for a new constitution. By then, the weakened provisional government had recognised Finland as a state in its own right but wanted to preserve the union along with some prerogatives. After the October Revolution, the majority non-socialist parties decided to cut all ties with the new Bolshevik Council of People's Commissars, and the Finnish Parliament passed the Power Act again in November, effectively declaring Finland sovereign. The Social Democrats also sought independence but wanted to have the Bolsheviks recognise it first. Finland was declared an independent republic officially on December 6, 1917.