A classified catalogue is a form of subject catalogue, in which the entries are arranged by the class number which representing the subject of document, according to the classification scheme chosen for a library. Let us examine a few formal definitions of a classified catalogue.
a) A catalogue in which some entries are number entries and some are word entries (Ranganathan).
b) A classified catalogue is a subject catalogue in which the entries are arranged by some recognized system of classification; the notation or classification symbol furnishes the method of arrangements (Margaret Mann).
c) A classified catalogue arranges its entries in a systematic order of subjects, the order usually being that of the classification scheme used for the arrangement of books on the shelves (Henry Sharp).
These definitions unambiguously indicate that a classified catalogue arranges its entries by the class number of documents. The class numbers are obtained by the application of classification system of a library. Ranganathan undoubtedly implied the class number of a document when he says that some entries are numbers entries. He was also clear that the classified part to be supported by an alphabetical index which comprises word added entries (except cross reference entry). The other two definitions do not refer to the bipartite nature of the classified catalogue. But without an alphabetical index or part, the classified catalogue is not able to fulfil the functions of a library catalogue effectively.
The classified catalogue, has therefore two parts. The classified part and the alphabetical part. The alphabetical part serves as an index and comprises of author, title, if necessary, subject, collaborators, series, and different types of cross-reference index entries.