Publishers, audio and visual producers and organizations housing a publishing arm may purchase ISBNs. For publishers with a US address, only the United States ISBN Agency may assign these numbers. ISBN's must be purchased in blocks and cannot be resold because one purpose of the numbers is to identify publishers in national and international databases. All titles and media products should receive an ISBN, and once assigned, these numbers should be registered with R.R. Bowker.
ISBNs should be located on the reverse of the title page. This location typically is also the copyright page, though ISBNs are not legal identifiers or otherwise related to the copyright. Alternatively, the ISBN may be placed on the bottom of the title page if placement on the reverse page is not feasible. On the back cover, the ISBN should sit above the bar code. For electronic publications, the ISBN should appear either on the title screen or the title screen's equivalent for that document. For products such as DVDs, the ISBN must be located on the item's label or, if that location is not feasible, in the bottom area of the packaging.
A publication produced in multiple media may retain the same ISBN if all the versions are packaged together; however, if packaged or distributed as separate publications, each must receive a unique ISBN. In either case, the location rules as stated above still apply. Finally, the format (such as paperback or hardback) should be indicated in brackets behind the number if the product appears in multiple forms.
Before 2007's ISBN-13, these numbers were ten digits long. The ISBN-13 provides a code for publisher locations, specific publishers, title and edition information and a validation number. Nevertheless, both the ten and 13 digit versions follow the same location rules. Bar code numbers located below the code are are not the same identifier as the ISBN; thus, ISBNs should always be situated above the bar code to avoid confusion between the two numbers.