The New Encyclopaedia Britannica : volume 20
Encyclopaedia Britannica Inc., 1988
 Buku Referensi
The New Encyclopaedia Britannica : volume 20, micropaedia konowledge in depth
Encyclopaedia Britannica Inc., 2002
 Buku Referensi
The New encyclopaedia Britannica : volume 20, macropaedia knowledge in depth
Encyclopaedia Britannica Inc., 1998
 Buku Referensi
Encyclopaedia Britannica : volume 20
Encyclopaedia Britannica Inc., 1967
 Buku Referensi
Encyclopaedia Britannica : volume 20
Encyclopaedia Britannica Inc., 1968
 Buku Referensi