A person new to book collecting wrote the IOBA asking what the above meant when seen in a catalogue entry. As many book buyers read this site, I thought I would share my reply with all of you. As I told him, however, this is a rule of thumb, and if you are serious about your book collecting, I recommend you find a dealer you trust and give him as much of your business as is humanly possible.
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Properly speaking, an edition of a book does not change through its various printings. For example, if I were to write a book, and the publisher printed 2500 as the first printing, and it sold out (a minor miracle, that, but I digress), and they printed another 2500, that would be the first edition, second printing. It is still the first edition until the text is substantially changed, or it includes new material (such as a forward, appendix, or cetera).
The ways various publishers have identified the various printings has, well, varied over the years, but over the last 20 years or so the industry has more or less decided to include a line of numbers, 1-0 ( such as 1 2 3 4 5 6 7 8 9 0 ) on the verso, or rear, of the title page. In the example given, the book has an intact number line, or all the numbers are present, so it is the first printing. The lowest number present is the printing of the book you have. An intact number line shows that you have a first printing.
The above is generally true, except when it is not. There are industry aberrations, such as Random House (but I understand their new titles conform).
Most collectors wish to be as close to the author as possible, so a first edition, first printing is generally (but not always) the most desirable book. Any reputable seller would only mean 1st edition, 1st printing when they simply say 1st edition.
Which brings up my next point; you may have noticed in this brief monologue I have noted 3 exceptions to the “rules of thumb” I have given you. If you are trying to learn about book collecting, you can do no better (and far worse) than to find a reputable bookseller who sells a lot of what you are interested in, ask her any questions you may have, and buy your books from her. We are generally a talkative lot, especially when the subject is books.
After all, as I have said before (like here & here), if you do not know books, you had better know your bookseller.