Letter line

A letter line is a convention occasionally used by publishers to denote the printing of a particular book. It is generally located on the copyright page and consists of a sequence of letters, the lowest alphabetically of which generally indicates the number of the printing (for example, "A" indicates a 1st printing, "B" a second printing, "C" a third printing, etc.).

Below is a letter line from Harvill Press, in this case indicating a first printing:
example of letter line

For more information on similar conventions used to identify printings, see also number lines.