The U.S. ISBN Agency has been assigning the 978 prefix to 13-digit ISBNs since 2007. In the previous article, we’ve explained the difference between 10- and 13-digit ISBNs and how both systems could be contained with the help of the 978 prefix. However, blocks of ISBN-13s built on existing ISBN-10s are exhausted. Therefore, the U.S. ISBN Agency has begun assigning ISBNs starting with the prefix element 979.