A series of numbers appearing on the copyright page of a book, where the lowest number generally indicates the printing of that particular copy (e.g., a "1" would mean a first printing, and a "29" would indicate a 29th printing).
The following example is of a book that is in its 4th printing from Citadel Press. Note that despite the apparent declaration of "First printing 1997" the number line indicates the actual printing.
Often, the number line does not appear in sequence, as shown below in this first printing from Macmillan. However, the lowest number still indicates the printing:
There may also be times when a number line also contains intended years of publication as well as printings. This number line, for example, describes a first printing, published in 1989 by Harper & Row:
Random House is a notable exception, where for a period of several decades a first printing was indicated with a number line that began with "2", often accompanied by the words "First Edition".
Still other times, publishers may choose to use a letter line rather than a number line, as here shown in a first printing from Harvill Press (the "A" being indicative, rather than a "1"):
A more in-depth article on number lines can be found on our blog.