Hidy, Ralph
Russell And Russell, 1949
Buku Teks
Phillips, Ralph W.
United states Government Printing Office, 1945
Buku Teks
Tyler, Ralph W.
University of Chicago, 1949
Buku Teks
Byrns, Ralph T
Scott Foresman, 1984
Buku Teks
Byrns, Ralph T.
Illinois, 1984
Buku Teks