Columbia subject headings:
Coal miners--United States.
Coal mines and mining--United States.
Labor productivity--United States.
Wages--United States.
Harvard subject headings:
Coal miners--United States
Coal mines and mining--United States
Labor productivity--United States.
Wages--United States.
Princeton subject headings:
Coal miners--United States.
Coal mines and mining--United States.
Labor productivity--United States.
Wages--United States.