Columbia subject headings:
Harvard subject headings:
Concrete construction--History--20th century.
Fortification, Field.
Military engineering--History--20th century.
World War, 1914-1918--Campaigns--Western Front
Princeton subject headings:
Concrete construction--History--20th century.
Fortification, Field.
Military engineering--History--20th century.
World War, 1914-1918--Campaigns--Western Front.