Columbia subject headings:
Harvard subject headings:
Earth (Planet)--History--21st century--Outlines, syllabi, etc.
Earth (Planet)--Miscellanea
History, Modern--20th century--Outlines, syllabi, etc.
Twentieth century--Outlines, syllabi, etc.
Twenty-first century--Outlines, syllabi, etc.
Princeton subject headings:
Almanacs.
Encyclopedias.
History, Modern--20th century--Encyclopedias.
Twentieth century--Encyclopedias.
Twenty-first century--Encyclopedias.