is essential in subject work becausesome subjects are closely related andyet not sufficiently so to be filed to-gether.
The use of the cross reference sheet
A portion of the reason why Luhmann chose a duplex numeric filing system for his zettelkasten.
ne of the chief drawbacks in using the Alpha-betic and Numeric systems in conjunction with asubject file is that in handling a large amount ofmaterial a great deal of cross referencing has tobe done, causing much extra labor and filling upthe files.
drawbacks of alphabetic and numeric systems with respect to cross referencing