ISBN Number

'The International Standard Book Number (ISBN) is a unique
 numeric commercial book identifier based upon the 9-digit
 Standard Book Numbering (SBN) code created by Gordon Foster,
 Emeritus Professor of Statistics at Trinity College, Dublin, for
 the booksellers and stationers W. H. Smith and others in 1965.'
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When designing my book covers I was looking at existing books
 to see what was needed,
all books have a unique ISBN number which is normally written
 above the barcode.