Union of Horodło in the context of "Castellans of the Polish–Lithuanian Commonwealth"

Władysław III of Poland (31 October 1424 – presumably 10 November 1444), also known as Ladislaus of Varna, was King of Poland and Supreme Duke of the Grand Duchy of Lithuania from 1434, as well as King of Hungary and Croatia as Vladislaus I from 1440 until his presumed death at the Battle of Varna. He was the eldest son of Władysław II Jagiełło (Jogaila) and the Lithuanian noblewoman Sophia of Halshany.

Władysław succeeded his father shortly before turning ten in 1434 and was, therefore, deemed unfit to rule until coming of age. Cardinal Zbigniew Oleśnicki acted as regent and a temporary provisores council executed power in the king's name. However, Władysław's legitimacy to the crown was contested by Lesser Polish nobles favouring the candidacy of Siemowit V of Masovia, who was of Piast lineage. In the aftermath of the coronation, Spytko III of Melsztyn accused Oleśnicki, the council and the Catholic Church of exploiting the king's youth to hold authority. A sympathiser with the Czech Hussites, Spytko was killed at the Battle of Grotniki in 1439, thus ending the hostilities.

The Lithuanian nobility (Lithuanian: bajorija) or szlachta of the Grand Duchy of Lithuania (Lithuanian: Lietuvos Didžiosios Kunigaikštystės šlėkta, Polish: szlachta Wielkiego Księstwa Litewskiego) was historically a legally privileged hereditary elite class in the Grand Duchy of Lithuania and Polish Lithuanian Commonwealth (including during period of foreign rule 1795–1918) consisting of Lithuanians from Lithuania Proper; Samogitians from Duchy of Samogitia; following Lithuania's eastward expansion into what is now Belarus, Ukraine and Russia, many ethnically Ruthenian noble families (boyars); and, later on, predominantly Baltic German families from the Duchy of Livonia and Inflanty Voivodeship.

Initially, the privileged social group of the Grand Duchy of Lithuania was called boyars. Boyars became part of the szlachta (nobility) during the Union of Horodło on October 2, 1413, initiating nobility in the Grand Duchy of Lithuania following the Western European model (with a hereditary system of heraldic identification), as well as an increase in the position of the Greater Lithuanian nobility. The Grand Duchy of Lithuania adopted Polish institutions of castellans and voivodes, and 47 selected boyars of Grand Duchy of Lithuania of the Catholic faith were adopted by Polish noble families and received Polish coats of arms.