Ron White recommends taking notes on 3 x 5 inch index cards. One should place the Dewey Decimal or Library of Congress catalog number in the upper left of their bibliography card and in the upper right corner one should number their cards consecutively (1, 2, 3, etc.). White indicates the importance of these numbers is primarily that they are unique, presumably so one can refer to them or reorder them if they are put out of order. (p46-7)