Printer's key

The convention appears in titles published around the middle of the 20th century; it became common practice after 1970.

[1][2] An example follows: This is how the printer's key may appear in the first print run of a book.

[4] The series may be in descending or ascending sequence: In some cases, rather than follow in unidirectional sequence, the numbers may alternate from left to right: In other cases, number lines may include a date segment consisting of two-digit consecutive-year codes: This indicates a second printing (or second impression) and that it occurred in 1970.

[6][7] With each successive reprint, the publisher needs to instruct the printer to change the impression character.

[11][12] In modern computer to plate typesetting and digital printing the entire printer's key may be easily re-imaged (instead of reset by erasing or removing individual characters).