Pick up the nearest paperback and turn it over. The barcode tells you more than you might expect once you know where to look.
The thirteen digits
Take 978-1-56581-231-4 as an example. The hyphens are for people; the bars do not carry them.
- Prefix (978 or 979). Marks the number as a book. 978 is the original range; 979 was opened when 978 began to run short. A 979 ISBN has no ten-digit equivalent, which matters only to very old systems.
- Registration group (1). The language or country area. 0 and 1 are English-speaking, 2 French, 3 German, 4 Japan, 5 Russia and so on. Some groups are several digits long.
- Registrant (56581). The publisher. Big publishers have short registrant numbers with room for many titles; small publishers get longer registrant numbers and fewer titles per block. A single ISBN bought from an agency has the longest registrant element of all.
- Publication (231). The specific book, within the publisher's block.
- Check digit (4). Calculated from the other twelve. Scanners use it to catch misreads; validators use it to catch typos.
The group and registrant parts vary in length, which is why you cannot hyphenate an ISBN correctly without the agency's range tables. That is also why ScanSpine keeps the hyphenation you enter rather than guessing.
Checking the check digit
Multiply the first twelve digits alternately by 1 and 3, add them up, and the check digit is whatever makes the total a multiple of ten. For 978156581231: 9+21+8+3+5+18+5+24+1+6+3+3 = 106, so the check digit is 4. If someone gives you an ISBN that fails this test, a digit is wrong.
The ten-digit form
Older books carry a 10-digit ISBN. Any 978 number can be converted: drop the 978, recompute the check digit using a different formula (weights 10 down to 2, modulo 11, where 10 is written as X). ScanSpine accepts either and stores the 13-digit form.
The five-digit add-on
The small barcode to the right is the price. 51699 is US $16.99, 90000 is "price not specified". The first digit is a currency code; the remaining four are the price in minor units. A price over $99.99 does not fit, so 59999 is used by convention. More in ISBN vs barcode.
What the barcode does not tell you
The barcode identifies the book. It does not identify the copy, the print run, the store it was sold in, or whether anyone ever scanned it. Retail systems resolve the number against their own catalogue; the author sees none of that.
A QR code beside the barcode fills that gap for readers, if not for registers. A reader's phone can follow it to a page you control, and if the code is dynamic, you can see where and when it was scanned. That is the half of the back cover ScanSpine is built around.