A library catalogue, we know, is a list of all or some of the holdings of a library. If two or more libraries make out such a list we call it a Union Catalogue. Larson defines a union catalogue as a "catalogue listing in one sequence the holdings or part of the holdings of two or more libraries."
Ranganathan defines a union catalogue as a "List of all the documents in two or more libraries giving the names of all the libraries where copies of each document can be found. A union catalogue may cover all kinds of documents or any restricted kind of them". Union catalogue is a very good example of cooperative cataloguing where two or more libraries cooperate with each other in listing their holdings partially or fully. Union catalogues can be prepared at local, regional, national and international levels.
Union catalogue, once prepared efficiently, serves several purposes. Some of which are: