Columbia subject headings:
Coal-mining machinery--Social aspects--United States--History--20th century.
United Mine Workers of America--History--20th century.
Harvard subject headings:
Coal-mining machinery--Social aspects--United States--History--20th century.
United Mine Workers of America--History--20th century.
Princeton subject headings:
Coal-mining machinery--Social aspects--United States--History--20th century.
United Mine Workers of America--History--20th century.