Dynastic union

A dynastic union is a type of federation with only two different states that are governed under the same dynasty, with their boundaries, their laws, and their interests remaining distinct from each other. It differs from a personal union in that a personal union is specifically under one monarch, but a dynastic union requires two monarchs.
Union of Kingdom of Aragon and Kingdom of Navarre.
With the assassination of Sancho IV, Navarre was invaded by his cousins Alfonso VI of Castile and Sancho V Ramirez of Aragon, and the latter was made king in 1076, which led to more than half a century (1076–1134) of Aragonese control.
Union of Kingdom of Aragon and County of Barcelona.
Marriage of Count of Barcelona Raymond Berengar IV of Barcelona and future Queen of Aragon Petronila of Aragon in 1137 that formed the Crown of Aragon.
Union of Crown of Castile and Crown of Aragon.
Marriage of Isabella I of Castile and Ferdinand II of Aragon in 1469 that laid the foundations for the kingdom of Spain. They did not ascend to their respective thrones until 1474 and 1479 respectively.
Union of Kingdom of Spain and Kingdom of Portugal.
Dynastic union between Spain (the union between the Crowns of Castile and Aragon) and Portugal (1580–1640), generally called the Iberian Union by modern historians, under the Philippine Dynasty.
Union of Grand Duchy of Lithuania and Kingdom of Poland.
Marriage of Jogaila and Queen Jadwiga of Poland on 1385, generally called the Union of Krewo. That union laid the foundations for the eventual formation of the Polish–Lithuanian Commonwealth.
Union of Kingdom of France and Kingdom of Navarre.
Following Salic law, Henry III, King of Navarre, a member of the House of Bourbon, succeeded to the French throne in 1589 upon the extinction of the male line of the House of Valois. Both houses were cadet branches of the Capetian dynasty, the ruling house of the kingdom of France since 987.
Union of Kingdom of England and Kingdom of Scotland.
When Elizabeth I of England died in 1603, the heir to the English Throne was King James VI of Scotland. Generally called the Union of the Crowns, this dynastic union was in place from 1603 until 1653 (when the monarchy was officially abolished) and again from 1659 until the two nations were politically united in 1707.