The printer’s key, also known as the number line, is a line of text printed on the copyright page (the verso of the title page) of books, used to indicate the print run. Publishers started to use this convention around the middle of the 20th century. An example follows: This is how the printer's key will 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".