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An International Standard Book Number (ISBN) is a unique number identifier that is assigned to each edition of a book. The ISBN is not only confined to printed material, but is assigned to each version of publication of a book that exists. The same book as an e-book, paperback, hardcover and other edition types would each have a different ISBN.
A unique ISBN is assigned to a book upon publication. Originally, ISBNs were 10 digits long, but starting in 2007 the codes were changed to 13 digits. The 13-digit ISBNs have a three-digit country code, which was not present before.
ISBNs are also accompanied by a bar code that contains certain information about a book. It is not uncommon for a book to have no ISBN in case of an author not following the proper ISBN procedures or if the book was published privately, but an ISBN can be assigned to the later versions without much effort.