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Zaolzie was an area disputed between Poland and Czechoslovakia, west of Cieszyn. The term "Zaolzie" is used predominantly in Poland (literally meaning "lands beyond the Olza River") and also commonly by the Polish community living on this territory. In Czech it is more frequently referred to with the term České Těšínsko/Českotěšínsko or by the neutral Těšínsko and Těšínské Slezsko (meaning Cieszyn Silesia). Zaolzie was made up of the former districts of Těšín and Fryštát and since the 1960 reform of administrative divisions it has been made up of Karviná District and the eastern part of Frýdek-Místek District. It is de facto eastern part of the western portion of Cieszyn Silesia.

Historically, the largest ethnic group inhabiting this area were the Poles. Under Austrian rule, the Cieszyn area was divided into four districts. One of them, Friedeck, had a mostly Czech population, the other three were mostly inhabited by Poles. During the 19th century the number of Germans grew. After decline at the end of the 19th century, at the beginning of the 20th century and later from 1920 to 1938, the Czech population grew significantly (mainly as a result of immigration and the assimilation of locals) and Poles became a minority, which they are to this day. Another significant ethnic group were the Jews, but almost the entire Jewish population was exterminated during World War II.