For example, here is the barcode from the back of one of my books.
So why do some companies charge so much simply to give you a barcode version of a number? Because unfortunately there are people out there who will pay the money without realizing they don't have to.
If you already have an ISBN number for your book, you HAVE the unique number. Heck, you could draw your barcode by hand, with a ruler, making the lines the right thicknesses if you had a chart telling you how thick and thin each line should be. There's no reason to pay someone $25 to do that for you.
Here is a website that offers free barcodes, so you can see how it works.
The format you need for books is called EAN-13. It's a style of barcode. Here is a software package that generates it for you -
I'll note again that most paperback publishers nowadays will automatically generate the bar code for you, for the back of your cover, so you don't have to worry about making one. Still, it's good to know how to do it, just in case.
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