The next Complex method in order, is the Numeric,which may be divided into three classes, straight num-eric, duplex and decimal. It is safe to say that withthe straight Alphabetic or Geographic, ninety-five per-cent of the cases where an Index is used will be moreefficiently handled by the use of either one of theseMethods, than by the Numeric. However, there aresome cases where there is a great deal of cross refer-ence, thus making the use of the Numeric methodmore advantageous.
This is likely the reason why most commonplacers using index card systems use alphabetic set ups by subject rather than Niklas Luhmann's duplex numeric variation.