| To explain the nature of this change to the ISBN, you need to know a bit about the internal structure of the 10-digit International Standard Book Number. A 10-digit ISBN consists of 4 elements, in this order: a group element, a publisher prefix, a title element, and a check digit. Example: ISBN 1-85375-390-4. When a barcode is applied to the publication, as is required by retail systems throughout the supply chain, the 10-digit ISBN has to become compatible with the 13-digit EAN-UCC international product coding system for bar codes. This is done by adding the 3-digit EAN product code for books "978" in front of the 10-digit ISBN and recalculating that ISBN's check digit to include those extra 3 digits. This 13-digit combination of the EAN 978 product code and an ISBN with a recalculated check digit was sometimes referred to as the "Bookland EAN". EXAMPLE: A 10-digit ISBN 0-901690-54-6 looks like this when it becomes part of the 13-digit barcode on a publication: 
To expand the capacity of the ISBN system, we added another EAN product code, "979", into the mix which opens up a new range of slightly less than 1 billion numbers. (Some of the numbers available in a particular group of the "979" range have already been assigned to printed music publications so those numbers can not be re-used as ISBN). Because both the "978" and "979" EAN product codes will be used for book products, the EAN product code has to become an integral element of the ISBN to prevent any possible confusion between duplicate numbers in the "978" and "979" ranges of ISBN. That turns the ISBN from a 10-digit into a 13-digit number. It also means that its check digit will be calculated over the preceding 12 digits (instead of 9). The new 13-digit ISBN format is identical to the barcoded version of a 10-digit ISBN. | 
