ISBN's

I know the International Standard Book Number (ISBN) is a way to identify a book by using the nine digit ISBN Code. However,
what I don't undestand is when I'm adding books here to my Library sometimes there are two ISBN's to chose from. I know one must be for the Paperback version of the book & the other one must be for the Hardback version of the book, right???? So my question is this, if I'm adding a Paperback or Hardback version of the book to my Library here, which ISBN do I use??? The first one or the second one??? If I knew which of the two ISBN's was for Paperback & Hardback it wouldn't be a problem for me.

Beatles1964

Look in your book. The ISBN info on the copyright page will indicate which ISBN is for the hardcover and which is for the paperback.

For instance, my edition of Umberto Eco's Turning Back the Clock says:
ISBN 978-0-15-101351-7
ISBN 978-0-15-603421-0 (pbk.)

So I know to use the second number because I own the paperback edition.

However, you may be talking about the fact that some books show both a 10-digit and a 13-digit number. That is due to the increasing number of books, so the system has switched to a 13-digit number and some books have both. In that case, you can use either; they're both for the same edition.

Ok, lilithcat that does esplain it for me. And you're right, I have also noticed both a 10 digit &
a 13 digit ISBN as well. I never knew which one of those I should use when I'm cataloging my
books.

Beatles1964