We’ve all seen it before, and probably a lot of us have even used it to find our textbooks. Our professors and syllabi often state it, and we always see it when we are checking out of an online bookstore…. but what *exactly* IS an ISBN number?

For those who are completely unfamiliar, the ISBN number stands for ‘International Standard Book Number’ and it is a unique identifier assigned to every book to help identify that book. There are actually two different types of ISBN numbers, the ISBN-10 consisting of 10 alpha-numeric characters and the ISBN-13 which has 13 alpha-numerics. Using this number anyone can easily find the textbook they are looking to buy on online bookstores like Textbook Nova and many others; and if you do not use the ISBN lookup feature then you can at least use this number to be confident that you found the correct textbook that is required for your class. Fortunately every new edition and format (ebook, hardcover, softcover, etc) is assigned a new ISBN, so if you are using an ISBN lookup to find your textbooks you can be positive you got the right edition.

**History**

In 1967 Gordon Foster realized that we needed a system to uniquely identify books, and a nine-digit number was invented called the Standard Book Numbering (SBN) system. However, shortly after the International Organization for Standardization (commonly known as ISO) published an article called ISO 2108 in 1970 which introduced the ISBN-10 we know of today. Fortunately they were smart enough to realize that a lot of books would be missing from this index, so any book that existed before the ISBN was introduced can still be referenced by it’s nine-digit SBN number preceded by a zero (thus the ten characters required to make a valid ISBN-10.

Any textbook that was assigned an ISBN number assigned after January 1st, 2007 was assigned an ISBN-13 number.

**What makes an ISBN?**

Every ISBN number consists of at least four parts, and ISBN-13 will include a prefix “EAN” number that is provided by GS1 which is an international non-profit organization. As of this writing the only “EAN” numbers that have been provided are 978 and 979, but as more books are published eventually they will assign a new EAN number for those books.

The next 4 numbers specifies the “Registration Group Element” which is a language specific code that identifies the country or territory that the textbook was registered.

Then there is the “Registrant Element” or the publisher code. The national ISBN agency keeps track of all the publishers and has assigned each of them a specific code that they use. At this time they have more than 900,000 registered publisher codes.

The title of the book is the next section also called the “Publication Element”. Large book publishers will often receive a block of ISBN codes and when they publish a book they assign the title of the book to a specific Publication Element code.

Finally there is a “Checksum” which is a number that is calculated to validate the ISBN number.

**Validation of an ISBN**

If you are having trouble doing an ISBN lookup for a textbook it could be for one of 3 reasons… Either 1) You are typing the wrong number into the search bar, 2) The search you are using does not work or 3) The person who gave you the ISBN number typed it wrong. If you suspect that the ISBN number you have is not valid you can always calculate the checksum digit yourself on the number to validate it and make sure that there is actually a textbook with that assigned ISBN number. Here’s how:

For an ISBN-10 with ten digits:

- Multiply the very first digit by 10 (if this digit is the character ‘X’ it should be counted as 10)
- Multiply the second digit by 9 (again, if this second digit is ‘X’ count it for the value of 10)
- Multiply the third digit by 8
- Repeat this process by multiplying the next number by one less than the previous
- Once you get to the right most digit (the one that will be multiplied by 1), add all of these digits together.
- Now you will need to perform a Modulo, which is fancy-math term for “Find the remainder”. Take the large number you just added and divide it by 11 and find the remainder of that number. The result of your modulo operation should
**not**be a fraction, if you just did this in a calculator you just need to multiply that fraction by 11 to get the modulo. - Now that you have your modulo, subtract it from 11 (“11 – your result”).
- Finally perform one more modulo operation with your number to get the Checksum digit (remember if you ended up with 10 it should be considered X in this case)

For an ISBN-13 with thirteen digits:

- Every
*other*number is multiplied by 3, starting at the second digit. The easiest way to do this is by multiplying the second digit by 3, and adding it to the first digit. - Repeat this process until you are down to the last digit again (your next operation will be multiplying the fourth digit by 3 and adding it to the third digit.)
- Add all of these numbers together, and perform the Modulo 10 operation (See above for an example on how to perform the modulo, just remember we are doing modulo 10 here,
**not**modulo 11). - Finally subtract your result from 10 to get the Checksum digit (“10 – your result”). The ISBN-13 is special in that if you end up with the number 10 for your checksum digit, it is represented by the number 0 (unlike the ISBN-10 where 10 is represented by ‘X’).

Whew! That was a lot of math, but it’s over now! There is a lot more to learn about ISBN numbers, but hopefully this will give you an idea of why it is important and how it can be beneficial when you are buying textbooks for your classes. Remember you can always be confident that you have found the correct book if you use an ISBN lookup like the one provided by Textbook Nova. Please feel free to reach out if you have questions or any corrections to this post.