ISBN barcodes are based on the EAN symbology. The ISBN numbering system predates barcoding and was incorporated into the EAN system by the use of a "978" prefix.
From January 2007, ISBN numbers became 13 digits long, rather than the previous 10 digits.
The change was necessary so that the ISBN system have sufficient numbers available for future growth.
Previously, the ISBN number was always prefixed with the digits "978" when applying the number to an EAN barcode.
There will now be an added alternative ("979") prefix in order to expand the numbering system. This means that the full 13 digits must now be communicated.
The change also required that the 13-digit number should appear above the barcode.
Previous Book Labeling - Before January 1, 2007
The EAN-13 is displayed below (and encoded within) the bar code
Printing only the ISBN-10 above the bar code is appropriate for titles published before January 1, 2007.
Current Book Labeling - After January 1, 2007
There is no change to the EAN-13 bar code itself and the EAN-13 is still displayed below (and encoded within) it.
Printing only the ISBN-13 above the bar code is recommended for titles published after January 1, 2007.
During the transition period, a third format was used - mainly in North America: ISBN Dual
Dual Numbering - Printing Both ISBN-10 and ISBN-13 Above Bar Code
Dual numbering allows trading partners to identify books using both an ISBN-10 and an ISBN-13.
Dual numbering is valid only for ISBN-13s that begin with '978'.
There is no dual numbering for ISBNs that begin with '979'.
When displaying dual ISBNs above the bar code, publishers may display the ISBNs in the sequence they prefer (i.e., ISBN-13 above ISBN-10 - as shown in the example above - or ISBN-10 above ISBN-13 - not shown).
If dual numbering is used above the bar code, it should also be used everywhere else the book's identifier is referenced (i.e., the copyright page, catalogues, order forms, etc.).