Columbia subject headings:
Harvard subject headings:
Bottrop (Germany)--Emigration and immigration.
Bottrop (Germany)--Foreign population.
Foreign workers, Polish--Germany--Bottrop--History.
Foreign workers, Polish--Germany--Ruhr (Region)--History.
Polish people--Germany--Bottrop--History.
Polish people--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Emigration and immigration.
Ruhr (Germany : Region)--Foreign population.
Princeton subject headings:
Foreign workers, Polish--Germany--Bottrop--History.
Foreign workers, Polish--Germany--Ruhr (Region)--History.
Immigrants--Germany--Bottrop.
Immigrants--Germany--Ruhr (Region).
Polish people--Germany--Bottrop--History.
Polish people--Germany--Ruhr (Region)--History.