The printer's key, also known as the number line, is a line of text printed on a book's copyright page (often the verso of the title page, especially in English-language publishing) used to indicate the print run of the particular edition. The convention appears in titles published around the middle of the 20th century; it became common practice after 1970.
An example follows:
|1 2 3 4 5 6 7 8 9 10|
This is how the printer's key may appear in the first print run of a book. Numbers are removed with subsequent printings, so if "1" is seen then the book is the first printing of that edition. If it is the second printing then the "1" is removed, meaning that the lowest number seen will be "2".