"But paper was too expensive to throw away so casually; indeed, the undertaker (and in many cases the author) was generally expected to pay for it. All sheets of such valuable material had to be accounted for. Instead of a revise, the first sheets of a print run would therefore often be checked as the rest were being printed off. In such a case, books would inevitably be made up of sheets in different states of correction. The consequence was that no two final copies out of a given edition would necessarily be the same. Indeed, in its modern sense, the very concept of ‘edition’ is entirely anachronistic."

-Adrian Johns, The Nature of the Book:
Print and Knowledge in the Making