POST-COORDINATE INDEXING MODEL 

All the pre-coordinate indexing models discussed so far are of unidimensional in nature based on the order of significance. The significance order is again based on the syntactical rules of the given pre-coordinate  indexing model. Rigidity of significance order may not meet the approaches of all users with a particular pattern of coordination, even with permutations. Therefore, the need was felt for a multidimensional system to permit access to subjects from any point of view. The alternative naturally was to prepare an index, where the subject concepts will be  kept separately  uncoordinated by the indexer. Coordination of concepts could be done  at the time  of search. It should be  noted that the process of coordination of concepts exists in both the pre-and post-coordinate indexing models, but it is done at two different stages. The indexing system which has the ability to freely coordinate terms  at the  time of search, thereby providing an unlimited number of access points to a document belongs to the post-coordinate or coordinate indexing models. 

Broadly, there are two types of post-coordinate indexing systems -Term Entry System and Item Entry System. In the Term Entry System, index entries are made for a document under each of the appropriate terms and are arranged in alphabetical sequence. A card catalogue using unit cards is of this kind. In this system, the number of index entries for a document is dependent on the number of terms representing the content of the document. Here terms are posted on the item. Uniterm Indexing and Optical Coincidence Card are examples of Term Entry System. Item Entry System takes the opposite approach and makes a single index entry for each document, using a physical form which permits access to the entry from all appropriate terms. Here, items are posted on the term. Edge-Notched Card or Punched Card System is the best example of item entry system. 

Most important features of the post-coordinate indexing system  are : (a) No  sequence of terms or citation order is necessary. Every term gets equal weight; (b) constituent concepts of a composite subject are kept separately uncoordinated by the indexer and the coordination of concepts is done by the searcher at the search stage; (c) The searcher has wide options of the free manipulation of the classes at the time of searching. As a result, the searcher has the complete freedom to coordinate the index terms in any combination or order required; (d) Multidimensionality in relationships of concepts can be retained. 

Post-coordinate indexing system was first conceived by Mortimer Taube in 1953 as a means of organising the research reports acquired by the US Armed Services Technical Information Agency. This indexing system became  very  popular due to its simplicity. Taube's system was based upon "uniterm cards", a card-based system. The principles of this system have been adapted and further evolved in computer-based bibliographic information retrieval systems. When Taube conceived the system, it was based on unit term. Soon he changed the idea of unit term to unit concept.