Columbia subject headings:
Petroleum industry and trade.
World Petroleum Congress (3d : 1951 : The Hague)
Harvard subject headings:
Legislative materials.
Petroleum industry and trade
World Petroleum Congress ( 3d : 1951 : The Hague)
World Petroleum Congress (3rd : 1951 : The Hague)
Princeton subject headings:
Petroleum industry and trade.
World Petroleum Congress ( 3d : 1951 : The Hague)