Dating book editions
In the days of letterpress printing, where each character was a metal block, all the printer had to do was to pick out the relevant blocks from the "sheet"; then the stack of blocks, which had been laboriously laid out when the page was first set up, could be inked for the reprint.
With many of the larger publishing houses, you’re on the lookout for a string of numbers on the copyright page, as shown in this copy of : The string of numbers can be descending, as it is here, ascending, or in totally random order.
To identify what printing you have, look for the lowest number in the string.
In this example, the lowest number is 1, so I have a first edition, first printing.
If it is the second printing then the "1" is removed, meaning that the lowest number seen will be "2".
The purpose of this arrangement is to keep the line of numbers roughly centered even as the numbers are removed with subsequent printings.However, to most booksellers and collectors, a “true” first edition is both the first typeset version of the book AND the first printing.