What does it mean when a book has two ISBN numbers?

I was just wondering...I have quite a large book collection, and I've noticed that some of my books have more than one ISBN number. They look something like this on the back of the book: ISBN-13: has 13 numbers, always starts with a 9, and ISBN-10: has 10 numbers, and usually starts with either a 0 or a 1. This is only on some of my books; most have just one ISBN. Does anyone know what it means when there are two?|||On January 1, 2007, the global book industry began the transition to 13-digit ISBNs, slowly phasing out the use of 10-digit numbers. The change to 13 digits was needed in order to expand the numbering capacity of the ISBN system and alleviate numbering shortages in certain parts of the world.





However, if they choose, some publishers may display BOTH the ISBN-10 and the ISBN-13 numbers on their books, during the transition to ISBN-13. While it is preferable to print only ISBN-13 numbers, some publishers will continue dual numbering/printing until the marketplace fully completes the transition to ISBN-13--hopefully sometime in 2008. That's why we still see some books with two ISBN's.





If you had to choose a single number-format to list for promotional reasons, I would say choose the ISBN-13, because that's the number-format everyone is moving to. However, listing both numbers wouldn't hurt, until the transition is complete.|||I think I heard why they have the 10- and 13-digit ISBNs, but I forgot what they said. Both numbers take you to the same book.