A unique code assigned to books printed for the public in the years after 1967 to identify the publisher and the title. It’s similar to the SBN code invented the year before, which had 9 digits. An SBN became an ISBN by prefixing it with a 0. In 1967, the ISBN was a 10-digit code. After 2007, books were assigned a 13-digit code. Any 10-digit ISBN issued before then can be converted to 13 digits using an ISBN converter.