Also known as a “printer’s key” or “publisher’s code,” the number line is a string of numbers printed on the copyright page, and it is used to indicate the print run for the book. They are often printed in descending order (10 9 8 7 6 5 4 3 2 1) and the lowest number generally indicates the printing of that particular copy.
With each printing of a book, the publisher instructs the printer to remove the lowest number that indicated that run. The reasoning behind removing the lowest number from the string is that if the printer is only removing one number, they are less likely to make a mistake than if they are introducing a new number each time.
Publishers began using number lines in the early 1940’s, and there is still not an industry-approved, standardized system for this convention. Therefore, each publisher has its own form of identification.
Some publishers may use ascending number lines (1 2 3 4 5 6 7 8 9 10), others sometimes use letters, and to make matters worse, some publishers may leave the “First Edition” statement on the copyright page but modify the number line to indicate a later printing!
Here’s some industry examples of number line differences:
This is an example of a number line that would indicate the second printing of a book done in 1970:
2 3 4 5 6 73 72 71 70
Here’s a hypothetical example of a number line from an outsourced printer indicating the third print run of a book by Acme Printing Corp. done in 1996:
3 4 5 6 7 8 9 10 APC 00 99 98 97 96
Random House is a notable exception to the basic number line. For a period of several years, they indicated a first printing with a number line that began with “2”.
Anness Publishing uses a number line that reads 1 3 5 7 9 10 8 6 4 2. The 1 indicates that this is a first printing. This same number line in a third print run would look like this: 3 5 7 9 10 8 6 4.
The number line can provide some useful information if you know how to read it. When it all comes down to it, the best way to know that you are purchasing a true first edition, first printing is to get a guide to identifying first editions with descriptions of number lines and other identification points for each individual publisher.