Grand Duke of Lithuania in the context of House of Gediminas

The Union of Lublin (Polish: Unia lubelska; Lithuanian: Liublino unija) was signed on 1 July 1569 in Lublin, Poland, and created a single state, the Polish–Lithuanian Commonwealth, one of the largest countries in Europe at the time. It replaced the personal union of the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania with a real union and an elective monarchy, as Sigismund II Augustus, the last of the Jagiellons, remained childless after three marriages. In addition, the autonomy of Royal Prussia was largely abandoned. The Duchy of Livonia, tied to Lithuania in real union since the Union of Grodno (1566), became a Polish–Lithuanian condominium.

The Commonwealth was ruled by a single elected monarch who carried out the duties of King of Poland and Grand Duke of Lithuania, and governed with a common Senate and parliament (the Sejm). The Union is seen by some as an evolutionary stage in the Polish–Lithuanian alliance and personal union, necessitated also by Lithuania's dangerous position in wars with Russia.

In a strict sense, the Union of Krewo or Act of Krėva (also spelled Union of Krevo, Act of Kreva; Polish: unia w Krewie; Lithuanian: Krėvos sutartis) comprised a set of prenuptial promises made at Kreva Castle on 14 August 1385 by Jogaila, Grand Duke of Lithuania, in regard to his prospective marriage to the underage reigning Queen Jadwiga of Poland.

Though very limited in scope, the "Union of Krewo", in historiography, often refers not only to the particular document but to events of 1385–1386 as a whole. After the 1385 negotiations, Jogaila converted to Christianity, married Jadwiga, and was crowned King of Poland in 1386.

Jogaila (Lithuanian: [jɔˈɡâːɪɫɐ] ; c. 1352/1362 – 1 June 1434), later Władysław II Jagiełło (Polish: [vwaˈdɨswaf jaˈɡʲɛwwɔ] ), was Grand Duke of Lithuania beginning in 1377 and starting in 1386, becoming King of Poland as well. As Grand Duke, he ruled Lithuania from 1377 to 1381 and from 1382 to 1401, at which time he became the Supreme Duke of Lithuania in exchange for naming his cousin Vytautas as the new Grand Duke. Władysław II initially served as King of Poland alongside his wife Jadwiga until her death in 1399, and then the sole ruler until his own death in 1434.

Raised a Lithuanian polytheist, he converted to Catholicism in 1386 and baptized as Ladislaus (Polish: Władysław) in Kraków, married the young Queen Jadwiga, and was crowned King of Poland as Władysław II Jagiełło. In 1387, he converted Lithuania to Catholicism. His reign in Poland started in 1399, upon the death of Queen Jadwiga, lasted a further thirty-five years, and laid the foundation for the centuries-long Polish–Lithuanian union. He was a member of the Jagiellonian dynasty in Poland that bears his name and was previously also known as the Gediminid dynasty in the Grand Duchy of Lithuania. The dynasty ruled both states until 1572, and became one of the most influential dynasties in late medieval and early modern Europe.

Sigismund II Augustus (Polish: Zygmunt II August, Lithuanian: Žygimantas Augustas; 1 August 1520 – 7 July 1572) was King of Poland and Grand Duke of Lithuania, the son of Sigismund I the Old, whom Sigismund II succeeded in 1548. He was the first ruler of the Polish–Lithuanian Commonwealth and the last male monarch from the Jagiellonian dynasty.

Sigismund was elder of two sons of Italian-born Bona Sforza and Sigismund the Old, and the only one to survive infancy. From the beginning he was groomed and extensively educated as a successor. In 1529 he was chosen as king in vivente rege election while his father was still alive. Sigismund Augustus continued a tolerance policy towards minorities and maintained peaceful relations with neighbouring countries, with the exception of the Northern Seven Years' War which aimed to secure Baltic trade. Under his patronage, culture flourished in Poland; he was a collector of tapestries from the Low Countries and collected military memorabilia as well as swords, armours and jewellery. Sigismund Augustus' rule is widely considered as the apex of the Polish Golden Age; he established the first regular Polish navy and the first regular postal service in Poland, known today as Poczta Polska. In 1569, he oversaw the signing of the Union of Lublin between Poland and the Grand Duchy of Lithuania, which formed the Polish–Lithuanian Commonwealth and introduced an elective monarchy.

The Jagiellonian (US: /ˌjɑːɡjəˈloʊniən/ YAH-gyə-LOH-nee-ən) or Jagellonian dynasty (US: /ˌjɑːɡəˈ-/ YAH-gə-; Lithuanian: Jogailaičių dinastija; Polish: dynastia jagiellońska), otherwise the Jagiellon dynasty (Polish: dynastia Jagiellonów), the House of Jagiellon (Polish: Dom Jagiellonów), or simply the Jagiellons (Lithuanian: Jogailaičiai; Polish: Jagiellonowie; Latin: Iagellonidae), was the name assumed by a cadet branch of the Lithuanian ducal dynasty of Gediminids upon reception by Jogaila, the Grand Duke of Lithuania, of baptism as Ladislaus in 1386, which paved the way to his ensuing marriage to the Queen Regnant Hedwig of Poland, resulting in his ascension to the Crown of the Kingdom of Poland as Ladislaus II Jagiełło (initially ruling jure uxoris jointly with Jadwiga until her death), and the effective promotion of his branch to a royal dynasty. The Jagiellons were polyglots and per historical evidence Casimir IV Jagiellon and his son Saint Casimir possibly were the last Jagiellons who spoke in their patrilineal ancestors' Lithuanian language; however, even the last patrilineal Jagiellonian monarch Sigismund II Augustus maintained two separate and equally lavish Lithuanian-speaking and Polish-speaking royal courts in Lithuania's capital Vilnius. The Jagiellons reigned in several European countries between the 14th and 16th centuries. Members of the dynasty were Kings of Poland (1386–1572), Grand Dukes of Lithuania (1377–1392 and 1440–1572), Kings of Hungary (1440–1444 and 1490–1526), and Kings of Bohemia and imperial electors (1471–1526).

The personal union between the Kingdom of Poland and the Grand Duchy of Lithuania (converted in 1569 with the Treaty of Lublin into the Polish–Lithuanian Commonwealth) is the reason for the common appellation "Poland–Lithuania" in discussions about the area from the Late Middle Ages onward. One Jagiellon, Ladislaus III of Poland, briefly ruled both Poland and Hungary (1440–1444), and two others ruled both Bohemia and Hungary (1490–1526) and then continued in the distaff line as a branch of the House of Habsburg.

Poland was ruled at various times either by dukes and princes (10th to 14th centuries) or by kings (11th to 18th centuries). During the latter period, a tradition of free election of monarchs made it a uniquely electable position in Europe (16th to 18th centuries).

The first Polish ruler whose existence is not debatable was Duke Mieszko I, who adopted Christianity under the authority of Rome in the year 966. He was succeeded by his son, Bolesław I the Brave, who greatly expanded the boundaries of the Polish state and ruled as the first king in 1025. The following centuries gave rise to the mighty Piast dynasty, consisting of both kings such as Mieszko II Lambert, Przemysł II or Władysław I the Elbow-high and dukes like Bolesław III Wrymouth. The dynasty's rule over Poland ceased with the death of Casimir III the Great in 1370. In the same year, the Capetian House of Anjou became the ruling house with Louis I as king of both Poland and Hungary. His daughter, Jadwiga, later married Jogaila, the pagan Grand Duke of Lithuania, who in 1386 was baptized and crowned as Władysław II Jagiełło, thus creating the Jagiellonian dynasty and a personal union between Poland and Lithuania.

Henry III (French: Henri III, né Alexandre Édouard; Polish: Henryk Walezy; Lithuanian: Henrikas Valua; 19 September 1551 – 2 August 1589) was King of France from 1574 until his assassination in 1589 and, as Henry of Valois, King of Poland and Grand Duke of Lithuania from 1573 to 1575.

As the fourth son of King Henry II of France and Queen Catherine de' Medici, he was not expected to inherit the French throne and thus was a good candidate for the vacant throne of the Polish–Lithuanian Commonwealth, where he was elected monarch in 1573. During his brief rule, he refused to sign the Henrician Articles into law, which would recognise the szlachta's right to freely elect their monarch. The Henrician Articles became, nonetheless, deeply rooted in Poland's politics as all his successors accepted them. Aged 22, Henry abandoned Poland–Lithuania upon inheriting the French throne when his brother, Charles IX, died without issue.

Sigismund III Vasa (Polish: Zygmunt III Waza, Lithuanian: Žygimantas Vaza; 20 June 1566 – 30 April 1632N.S.) was King of Poland and Grand Duke of Lithuania from 1587 to 1632 and, as Sigismund, King of Sweden from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. Religiously zealous, he imposed Catholicism across the vast realm, and his crusades against neighbouring states marked Poland's largest territorial expansion. As an enlightened despot, he presided over an era of prosperity and achievement, further distinguished by the transfer of the country's capital from Kraków to Warsaw.

Sigismund was the son of King John III of Sweden and his first wife, Catherine Jagiellon, daughter of King Sigismund I of Poland. Elected monarch of the Polish–Lithuanian Commonwealth in 1587, he sought to unify Poland and Sweden under one Catholic kingdom, and when he succeeded his deceased father in 1592 the Polish–Swedish union was created. Opposition in Protestant Sweden caused a war against Sigismund headed by Sigismund's uncle Charles IX, who deposed him in 1599.

John III Sobieski (Polish: Jan III Sobieski (Polish pronunciation: [ˈjan ˈtʂɛt͡ɕi sɔˈbʲɛskʲi]); Lithuanian: Jonas III Sobieskis (Lithuanian pronunciation: ['joːnäs so'bʲɛskis]); Latin: Ioannes III Sobiscius (Latin pronunciation: [joˈannɛs soˈbiʃiʊs]) 17 August 1629 – 17 June 1696) was King of Poland and Grand Duke of Lithuania from 1674 until his death in 1696.

Born into Polish nobility, Sobieski was educated at the Jagiellonian University and toured Europe in his youth. As a soldier and later commander, he fought in the Khmelnytsky Uprising, the Russo-Polish War and during the Swedish invasion known as the Deluge. Sobieski demonstrated his military prowess during the war against the Ottoman Empire and established himself as a leading figure in Poland and Lithuania. In 1674, he was elected monarch of the Polish–Lithuanian Commonwealth following the sudden and unexpected death of King Michael.

Stanisław II August (born Stanisław Antoni Poniatowski; 17 January 1732 – 12 February 1798), known also by his regnal Latin name Stanislaus II Augustus, and as Stanisław August Poniatowski (Lithuanian: Stanislovas Augustas Poniatovskis), was King of Poland and Grand Duke of Lithuania from 1764 to 1795, and the last monarch of the Polish–Lithuanian Commonwealth.

Born into wealthy Polish aristocracy, Poniatowski arrived as a diplomat at the Russian imperial court in Saint Petersburg in 1755 at the age of 22 and became intimately involved with the future empress Catherine the Great. With her aid, he was elected King of Poland and Grand Duke of Lithuania by the Sejm in September 1764 following the death of Augustus III. Contrary to expectations, Poniatowski attempted to reform and strengthen the large but ailing Commonwealth. His efforts were met with external opposition from neighbouring Prussia, Russia and Austria, all committed to keeping the Commonwealth weak. From within he was opposed by conservative interests, which saw the reforms as a threat to their traditional liberties and privileges granted centuries earlier.

Gediminas (c. 1275 – December 1341) was Grand Duke of Lithuania from 1315 or 1316 until his death in 1341. Gediminas is traditionally considered to have been the founder of Vilnius, the capital city of Lithuania (see: Iron Wolf legend). During his reign, he brought under his rule lands streching from the Baltic Sea to the Black Sea. The Gediminids dynasty he founded and which is named after him came to rule over Poland, Hungary and Bohemia.

The Christianization of Lithuania (Lithuanian: Lietuvos krikštas) occurred in the late 14th and early 15th centuries, initiated by the Lithuanian royals Jogaila, King of Poland and Grand Duke of Lithuania, and his cousin Vytautas the Great. It signified the official adoption of Catholic Christianity by Lithuania, the last pagan country in Europe. However, Lithuania's first ruler to be baptised was Mindaugas in 1250s. This event ended one of the most complicated and lengthiest processes of Christianization in European history.

Sigismund I the Old (Polish: Zygmunt I Stary, Lithuanian: Žygimantas I Senasis; 1 January 1467 – 1 April 1548) was King of Poland and Grand Duke of Lithuania from 1506 until his death in 1548. Sigismund I was a member of the Jagiellonian dynasty, the son of Casimir IV and younger brother of Kings John I Albert and Alexander I Jagiellon. He was nicknamed "the Old" in later historiography to distinguish him from his son and successor, Sigismund II Augustus. Before ascending to the Polish and Lithuanian thrones, he was Duke of Głogów from 1499, Duke of Opava from 1501, and governor of Silesia from 1504 on behalf of his brother, King Vladislaus II of Bohemia and Hungary.

Sigismund was born in the town of Kozienice in 1467 as the fifth son of Casimir IV and his wife Elizabeth of Austria. He was one of thirteen children and was not expected to assume the throne after his father. Sigismund's eldest brother and rightful heir Vladislaus II instead became the King of Bohemia, Hungary and Croatia as the successor to George of Poděbrady in Bohemia and then to Matthias Corvinus in Hungary, thus temporarily uniting these kingdoms. When Casimir died, the Polish-Lithuanian realm was divided between the remaining two older sons, with John Albert being crowned King of Poland, and Alexander as Grand Duke of Lithuania. Alexander inherited Poland following John Albert's sudden death in 1501. Hence, Sigismund's reign only began when he succeeded Alexander to both titles in 1506 at the age of 39.

Casimir IV (Casimir Andrew Jagiellon; Polish: Kazimierz Andrzej Jagiellończyk [kaˈʑimjɛʂ jaɡʲɛ(l)ˈlɔj̃t͡ʂɨk] ; Lithuanian: Kazimieras Andrius Jogailaitis; 30 November 1427 – 7 June 1492) was Grand Duke of Lithuania from 1440 and King of Poland from 1447 until his death in 1492. He was one of the most active Polish-Lithuanian rulers; under him, Poland defeated the Teutonic Knights in the Thirteen Years' War and recovered Pomerania.

The Jagiellonian dynasty became one of the leading royal houses in Europe. The great triumph of his reign was bringing Prussia under Polish rule. The rule of Casimir corresponded to the age of "new monarchies" in western Europe. By the 15th century, Poland had narrowed the distance separating it from Western Europe and became a significant power in international relations. The demand for raw materials and semi-finished goods stimulated trade, producing a positive balance, and contributed to the growth of crafts and mining in the entire country. He was a recipient of the English Order of the Garter (KG), the highest order of chivalry and the most prestigious honour in England.