Personal union

A personal union is the combination of two or more states that have the same monarch while their boundaries, laws, and interests remain distinct. A real union, by contrast, would involve the constituent states being to some extent interlinked, such as by sharing some limited governmental institutions. Unlike the personal union, in a federation and a unitary state, a central (federal) government spanning all member states exists, with the degree of self-governance distinguishing the two. The ruler in a personal union does not need to be a hereditary monarch.
The term was coined by German jurist Johann Stephan Pütter, introducing it into "Elementa iuris publici germanici" (Elements of German Public Law) of 1760.
Personal unions can arise for several reasons, ranging from coincidence (a woman who is already married to a king becomes queen regnant, and their child inherits the crown of both countries; the King of one country inherits the crown of another country) to virtual annexation (where a personal union sometimes was seen as a means of preventing uprisings). They can also be codified (i.e., the constitutions of the states clearly express that they shall share the same person as head of state) or non-codified, in which case they can easily be broken (e.g., by the death of the monarch when the two states have different succession laws).
Because presidents of republics are ordinarily chosen from within the citizens of the state in question, the concept of personal union has almost never crossed over from monarchies into republics, with the rare exception of the president of France being a co-prince of Andorra. In 1860 Marthinus Wessel Pretorius was simultaneously elected as the president of Transvaal and Orange Free State and he tried to unify the two countries but his mission failed and led to the Transvaal Civil War.
Andorra.
Even though France is now a republic with a president and not a monarchy, it has nevertheless been in personal union with the neighbouring nominal monarchy (non-hereditary) of Andorra since 1278.
England.
"After 1707, see Great Britain below."
France.
Note: The point at issue in the War of the Spanish Succession was the fear that the succession to the Spanish throne dictated by Spanish law, which would devolve on Louis, "le Grand Dauphin" — already heir to the throne of France — would create a personal union that would upset the European balance of power; France had the most powerful military in Europe at the time, and Spain the largest empire.
Goryeo.
King Chungseon reigned as King of Goryeo in 1298 and 1308–1313 and as King of Shenyang or King of Shen from 1307 (according to the "History of Yuan") or 1308 (according to "Goryeosa") to 1316. At that time, Goryeo had already become a vassal of Yuan dynasty and the Yuan imperial family and the Goryeo royal family had close relationship by marriages of convenience. Because he was a very powerful man during Emperor Wuzong's reign, he could become the King of Shenyang where many Korean people lived in China. However, he lost his power in the Yuan imperial court after the death of the Emperor Wuzong. Because the Yuan dynasty made Chungseon abdicate the crown of the Goryeo in 1313, the personal union was ended. King Chungsuk, Chungseon's eldest son, became the new King of Goryeo. In 1316, the Yuan dynasty made Chungseon abdicate the crown of Shen in favour of Wang Go, one of his nephews, resulting in him becoming the new King of Shen.
Great Britain.
"Before 1707, see England and Scotland."
"After 1801, see United Kingdom below."
Saxe-Coburg and Saxe-Gotha.
In 1826, the newly created Duchy of Saxe-Coburg and Gotha was initially a double duchy, ruled by Duke Ernest I in a personal union. In 1852, the duchies were bound in a political and real union. They were then a quasi-federal unitary state, even though later attempts to merge the duchies failed.
Saxe-Weimar and Saxe-Eisenach.
The duchies of Saxe-Weimar and Saxe-Eisenach were in personal union from 1741, when the ruling house of Saxe-Eisenach died out, until 1809, when they were merged into the single duchy of Saxe-Weimar-Eisenach.
Schleswig and Holstein.
"Duchies with peculiar rules for succession. See the Schleswig-Holstein Question."
Schwarzburg-Rudolstadt and Schwarzburg-Sondershausen.
The duchies of Schwarzburg-Rudolstadt and Schwarzburg-Sondershausen were in personal union from 1909, when Prince Günther of Schwarzburg-Rudolstadt succeeded also to the throne of Schwarzburg-Sondershausen, until 1918, when he (and all the other German monarchs) abdicated.
Scotland.
"After 1707, see Great Britain above."
Spain.
Leon, Castile and Aragon
Spain
Wales.
"After 1542, see England above."