Biała (Vistula) in the context of Biała Krakowska

Bielsko-Biała (Polish: [ˈbjɛlskɔ ˈbjawa] ; Czech: Bílsko-Bělá; German: Bielitz-Biala), is a city in the Silesian Voivodeship, southern Poland, with a population of approximately 166,765 as of December 2022, making it the 22nd largest city in Poland, and an area of 124.51 km (48.07 sq mi). It is the core of the broader metropolitan area with around 335,000 inhabitants. It serves as the seat of the Bielsko County, Euroregion Beskydy, Roman Catholic Diocese of Bielsko–Żywiec and the Evangelical Church Diocese of Cieszyn.

Situated north of the Beskid Mountains, Bielsko-Biała is composed of two former towns which merged in 1951—Bielsko in the west and Biała in the east—on opposite banks of the Biała River that divides the historical regions of Silesia and Lesser Poland. The history of Bielsko dates back to the 13th century, while Biała was founded in the 16th century and obtained city rights in 1723. Despite the administrative separation, both towns effectively functioned as one urban area already in the 19th century. Industrialization, especially the textile and automotive industries, was of great importance for its development in the past. Between 1975 and 1998, the city was the seat of Bielsko-Biała Voivodeship and currently lies within the Silesian Voivodeship.

The Duchy of Teschen (German: Herzogtum Teschen), also Duchy of Cieszyn (Polish: Księstwo Cieszyńskie) or Duchy of Těšín (Czech: Těšínské knížectví), was one of the Duchies of Silesia centered on Cieszyn (Teschen) in Upper Silesia. It was split off the Silesian Duchy of Opole and Racibórz in 1281 during the feudal division of Poland and was ruled by Silesian dukes of the Piast dynasty from 1290 until the line became extinct with the death of Duchess Elizabeth Lucretia in 1653.

The ducal lands initially comprised former Lesser Polish territories east of the Biała River, which in about 1315 again split off as the Polish Duchy of Oświęcim, while the remaining duchy became a fiefdom of the Bohemian kings in 1327 and was incorporated into the Lands of the Bohemian Crown in 1348. While the bulk of Silesia was conquered by the Prussian king Frederick the Great in the Silesian Wars of 1740–1763, Teschen together with the duchies of Troppau (Opava), Krnov and Nysa remained with the Habsburg monarchy and merged into the Austrian Silesia crown land in 1849. The so-called "commander line" of the Habsburg-Lorraine dynasty, a cadet branch descending from Archduke Charles, Duke of Teschen, held the title "Duke of Teschen" until 1918.