Columbia subject headings:
Sahara--Commerce--History--19th century.
Sahara--Commerce--History--20th century.
Trade routes--Sahara--History--19th century.
Trade routes--Sahara--History--20th century.
Harvard subject headings:
Sahara--Commerce--History--19th century.
Sahara--Commerce--History--20th century.
Trade routes--Sahara--History--19th century.
Trade routes--Sahara--History--20th century.
Princeton subject headings: