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This is an information page. It is not a Wikipedia policy or guideline; rather, its purpose is to explain certain aspects of Wikipedia's norms, customs, technicalities, or practices. It may reflect varying levels of consensus.

Many Wikipedia articles have an International Standard Book Number (ISBN) alongside the bibliographical entry for each book. Clicking on the linked 10- or 13-digit number links to WP:Book sources. This shows whether the particular book is listed in a library catalogue, at WorldCat, at Google Books, at major booksellers, or at other websites.

An example of a bibliographical entry as a reference or a suggestion for further reading would look like this:

• Carr, Edward Hallett (1972). What is History?: The George Macauley Trevelyan Lectures Delivered in the University of Cambridge, January–March 1961. Harmondsworth; Ringwood: Penguin Books. ISBN 0-14-020652-3.

The ISBN might lack hyphens between the digits, or it might contain spaces between the digits. Either way it should provide the link to WP:Book sources. Note that different editions of the same book may have different ISBNs. Also, books published prior to the 1970s do not have ISBNs, unless subsequently reprinted.

• Special:BookSources is the page to which each ISBN links. It links to many sources for a book throughout the world. You can also manually enter an ISBN – but you should click the linked ISBN to WP:Verify that the link is good.
• Category:International Standard Book Number includes all Wikipedia articles and templates directly related to ISBNs.
• Use {{ISBN}} to add an ISBN to an article, or |isbn= inside {{cite book}} and similar citation templates. Wikipedia:ISBN describes in detail how to add an ISBN to an article.

• International ISBN Agency – coordinates and supervises the worldwide use of the ISBN system.
• Conversion tool: ISBN-10 to ISBN-13 and ISBN-13 to ISBN-10 from the ISBN agency. Also shows verifies correct hyphenation and whether an ISBN is valid or not. (NB "This ISBN Converter Tool only supports ISBNs allocated inside the USA and Australia".)