It is the arrangement of entries of documents by class number that gives the systematic and structured display of subjects in a classified catalogue. It maps out the subordinate and coordinate subject divisions and puts them in a logical sequence. This systematic arrangement to a certain degree reflects the logical thinking of specialists in different subjects and gets their appreciation. While displaying the entries in a classified catalogue, merely giving the class numbers would not be very helpful to users of the catalogue. The class numbers are not intelligible to persons who have no knowledge of the classification scheme chosen for the library. Therefore, while displaying the class numbers in guide cards, for entries under them, it is absolutely necessary to give their verbal equivalents of the divisions of class• numbers. These verbal equivalents -provided for class numbers in a classified catalogue are known as 'Feature Headings'. These headings are the translation of the class numbers of documents into their verbal equivalents, representing the different hierarchical levels of the subject. A very good example of the use of feature headings is the British National Bibliography. They are also useful as an obvious source for deriving subject index for the alphabetical part of the classified catalogue. It is useful and relevant here to quote the comments of Foskett on Feature Headings:
"Feature headings are verbal extensions, as well as being the major source of indexing terms, will indicate the terms which should be used for guiding, both in the catalogue and on the shelves. In general, libraries provide quite inadequate guides to their systematic arrangement, assuming that readers can find their way through the catalogue with a minimum of help. It may be thought extreme to make a guide for every feature heading, but this is what is done by BNB, and there is no doubt that it makes the systematic arrangement a great deal easier to use. Students should compare the careful guiding of BNB with say, American Book Publishing Record, in which the guiding is minimal, the superiority shows up very clearly in the cumulations. (A.C. Foskett: Subject Approach to Information, page 286).
Providing feature headings is theoretically sound and extremely useful and it is done quite well in a printed catalogue or bibliography like BNB. But in a card catalogue, the breakdown of the class numbers into their hierarchical divisions would need too many cards for each entry. It sometimes diminishes its practical utility. Therefore, it has to be done with considerable imagination and caution. Feature headings should not be allowed to defeat their very purpose.