I think I have some more insight into the number line mystery. While browsing books at the thrift store today, it came to me quite suddenly that I had some idea about the extra numbers found on this line in some books.

Not that it matters, but this particular line came from a Harry Potter book - specifically, The Prisoner of Azkaban.

The first part of this line indicates that this is the eighth printing of the book, as the lowest seven numbers have been removed.

It also happens that this book was published in 1999, and that's the key to the second part of the number line.

For instance, consider:

  10 9 8       9/9 0/0 1 2 3 4

The original number line would have looked something like this one:

  10 9 8 7 6 5 4 3 2 1   9/9 0/0 1 2 3 4

You'll notice that while the first seven printings have been removed from the number line, the second set of numbers is still there and has not changed! I'm fairly certain that this is because the second set of numbers indicates the year of printing.

It's not so apparent when you look at a single title, but it makes a whole lot more sense when you consider that the publisher likely uses a template for this page for all their titles. When the year changes, they simply remove the lowest year on the page - much like they do with the printing run. So:

  10 9 8       9/9 0/0 1 2 3 4

Indicates the eighth printing, which occured in 1999 - the 9/9 indicating first the decade and then the year, thus being the lowest (earliest) value in that section of the number line. Meanwhile the 0/0 1 2 3 4 indicates a decade of 0, and the numbers thereafter indicate the year within the decade - in other words, '00, '01, '02, '03 and '04. Essentially meaning nothing at this point, but allowing the publisher to create a template that will work for a number of years.

  10 9 8       9/9 0/0 1 2 3 4

Something printed this year might have a number line like this:

  5 7 9 10 8 6 4     04 05 06 07 08 09

Which indicates a fourth printing (with 4 being the lowest number in the "printing" section of the number line) and a printing year of '04, or 2004, with that being the lowest number in that section of the printing line. This particular template should be good through the year 2009.

To return to the earlier question, this:

  12 11 10 9 8 7 6 5    4 5 6 7 8/0

Should indicate a fifth printing (the first four runs missing from the list) that was printed in 2004. The slash-zero (/0) at the end indicates the decade, and the lowest number on the line (4) indicates the year within that decade. The number line for a first printing of this book should read something like this:

  12 11 10 9 8 7 6 5 4 3 2 1  4 5 6 7 8/0

Using this information, you ought to be able to establish the printing run and the printing year for any recent book. Of course, there are those that don't include this sort of information, and then all of this won't really help too much. Can't help with those. Sorry.

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