Cat. & Reference: The Printer’s Key
If you open up a book to the copyright page, you may notice a string of number which looks something like this:
1 3 5 6 9 10 8 6 4 2
This is called the “printer’s key” or “number line,” and it is used to mark the print run that a given physical book belongs to.
Publishers try to estimate how popular a book is going to be so that they don’t print too many copies and end up with extras. However, sometimes a book is more popular than they anticipate, so the book has to go into a second (or third or fourth) printing. A second printing is supposed to be virtually identical to the first one. There may be very minor corrections or changes, but the point is to be able to use the same plates that were used for the first printing. However, publishers still like to be able to keep track of which printing a book belongs to, so they add the printer’s key to the copyright page of the book. For each new printing, the lowest number is marked out, so that the new lowest number corresponds to the print run.
Sometimes the numbers increase from the outside in (as seen above), which allows the text to remain centered. Sometimes the numbers just increase from right to left. It’s also possible for the printer’s key to include a separate block of years, so that the publisher can keep track of when the book was printed.
Librarians don’t typically pay much attention to the printer’s key or to printing dates; if the book hasn’t changed enough to warrant a new edition, the catalog record from the original printing will still be accurate enough for our purposes. However, if a given printing of a book was particularly rare or valuable, that information might be included in a note.