International Standard Book Number

The ISBN is used to quickly index and find books. Because there is only one ISBN per book the chance of buying the wrong book is eliminated.

An ISBN looks like this: ISBN 1-55105-083-8.

The ISBN can be checked by the following, simple, algorithm: Multiply the first number by 10, the second by 9, the third by 8, until you get to multiply by 2. Add up the results of each multiplication, add the check digit, and divide it by 11. If there is no remainder, you have a valid ISBN.

The check digit '10' is written as 'X'. There is no need to add 11 or more, because that wouldn't help for dividing by 11 without a remainder.

Source o' knowledge:
Ground Control, ISBNs are assigned in "blocks". A large publishing company will own multiple blocks, and will be able to publish as many books as they want.

It isn't strictly true that there exists only one ISBN per book. Though it wasn't explicitly stated above, hardcovers and softcovers of the same title have different ISBNs. This is a fairly logical thing to presume, but there are other situations where publishers don't exactly play by the rules - books whose cover art has been modified during their runs (movie tie-in covers, book club editions and the like) sometimes have different ISBNs as well.

They shouldn't - generally when the new covers are released the old versions are no longer printed and the ISBNs should transfer from one edition to the next. The only reason to have two separate ISBNs for two practically identical books is if they're both in print at the same time (which would be nice, in my opinion - most people shopping at my bookstore despise tie-in covers with a passion. We wouldn't stock them if we had any choice) but they rarely are.

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