The barcode is made up of 12 digits, in various groups. The first
two show the country that issued the barcode. The next
four digits indicate the manufacturer. Some countries have a three
digit country code, so they only have three digits for the manufacturer
code.
The final six digits are a product code that the manufacturer assigns.
Every product has a separate code. Even different sized boxes of
the same product must have a different code.
There are some quirks in the system. Often books, which have the
code 978, actually start the code with 78, so that there are enough
digits left in the code to add the complete international book number.
Sometimes the missing nine is simply printed in front of the code.
