The International Standard Book Number(ISBN) is a unique numeric commercial book identifier. Publishers purchase ISBNs from an affiliate of the International ISBN Agency.An ISBN is assigned to each edition and variation (except reprintings) of a book. For example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, and 10 digits long if assigned before 2007. The method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country.
The initial ISBN configuration of recognition[clarification needed] was generated in 1967, based upon the 9-digit Standard Book Numbering (SBN) created in 1966. The 10-digit ISBN format was developed by the International Organization for Standardization(ISO) and was published in 1970 as international standard ISO 2108 (the SBN code can be converted to a ten-digit ISBN by prefixing it with a zero digit "0").
Privately published books sometimes appear without an ISBN. The International ISBN agency sometimes assigns such books ISBNs on its own initiative.
Another identifier, the International Standard Serial Number (ISSN), identifies periodical publications such as magazines; and the International Standard Music Number (ISMN) covers for musical scores.