(taken from: Guidelines for the Implementation of 13-Digit ISBNs, International ISBN Agency, Berlin 2004)
The new ISBN now consists of 13 digits: the 3-digit prefix that identifies the book industry (currently 978), followed by the core 9-digit number and the recalculated check digit that validates the internal integrity of the whole number. As such it is also identical to the EAN ‘Bookland’ 13-digit code that already appears encoded in the bar code printed on the back of the book.
The need arose for a new number structure for ISBN because the supply of numbers available to meet the growing number of publishers and publications worldwide was beginning to run out. When it became necessary – after January 2007 - the book industry can now use the additional 2 EAN prefix 979. When ISBN agencies in some parts of the world run out of their current allocation of ISBNs, they will be authorised to assign ISBNs with 979 prefixes. It is likely that, due to the availability of different publishers’ ranges, an agency may be issuing some ISBNs with the 978 prefix and others with the 979 prefix.
The methods by which 13-digit ISBNs was allocated will be different from current arrangements. It is most unlikely that publishers will be allocated their existing prefixes when 979-prefixed ISBNs are allocated. Therefore one cannot assume that 978 + publisher identifier will be the same as 979 + publisher identifier.
All these considerations made it essential that all parts of the industry were ready to work with 13-digit ISBNs from 1 January 2007, as all books published after that date must now carry 13-digit ISBNs.