Library catalogue in the card form is by far the most popular physical form. It is widely prevalent in libraries throughout the world including India. In this form the bibliographical elements of every document are recorded on a single card. This method of representing every document on a single card is known as the unit card principle. These cards stand in card-trays or cabinets with a punched hole of about half a centimeter from the bottom for inserting a locking rod. This locking system keeps the cards from falling out and also prevents unauthorised persons from removing any card from the tray. Because of its wide usage all over the world, many aspects pertaining to the card catalogue are standardised. For example, 12.5 x 7.6 cm. is the universally adopted size for a catalogue card. Similarly, the sizes of cabinets, trays for a card catalogue are all of uniform standard. Consequently, most of these items of furniture could be obtained readily from commercial vendors.
Some of salient features, which made the card catalogue quite popular are:
a) It is flexible in keeping it constantly updated.
b) The users and the library staff can handle it with ease.
c) The cards are single, self-contained units. This feature permits additional approach points and cross references in the catalogue.
d) The libraries using the card catalogue can participate in central and cooperative cataloguing scheme. This reduces the burden of the staff.
In fact, the unit card principle is one of the most beneficial outcomes of the card catalogue. This principle paved the way for centralised cataloguing of documents at central place. The printed unit cards can be multiplied and distributed to other libraries at a fairly low price. The Library of Congress, USA, was the pioneer in starting this card service and many libraries in the United States and other countries use this service. The well-known commercial bibliographic publisher, H.W. Wilson and Co. also provides printed catalogue card service to libraries, for selected items. Cooperation in compiling bibliographic records is another extension of centralised cataloguing. Libraries participating in cooperative cataloguing provide catalogue entries to the Library of Congress of those documents that are not available at the Library of Congress.
The Library of Congress gets them printed for distribution, as part of its catalogue card service. The British National Bibliography (now incorporated with the British Library) from its very inception in 1950, assumed responsibility as a national cataloguing agency. Besides its printed weekly and monthly editions and other cumulations, a printed card service is also made available on subscription to individual libraries.