23 25 27 29 30 28 26 24
I took that particular set from my copy of "The Great Gatsby." It's pretty and all, but...what the heck do those numbers MEAN?
I've pondered this question since I was a child, and you'd think that in the age of Google and Wikipedia you'd be able to find the answer, but you have to know what those numbers are CALLED before you can SEARCH for them. Amazingly I've discovered that they aren't called anything. They're just a "number row."
One of the local Old Goats finally let me in on the number row secret: those numbers reveal the current edition of the book; in the case of the above example, my copy of "The Great Gatsby" is the 23rd edition printed by that particular publishing company: the lowest digit in the row is the edition number.
You might wonder why the copyright page doesn't just say "Edition 23," or why the numbers alternate and are centered, or why different books use different conventions; some are left- or right-justified, some include the year of the edition (eg. "06 07 08 09 10 5 4 3 2 1"), and some dispense with the system altogether. To understand this you need to understand why they're there in the first place.
The reason for the number row is because books are re-printed from a set of plates. When you re-use a set of plates to print a new edition of a book -- because the last one sold out -- you don't want to recreate the copyright page so it just says "Edition 23." That would require a new plate for a relatively trivial change.
Far cheaper is to simply obscure the numbers on the plates for the editions you've already printed. The plate is originally set with all the numbers from one to ten (or ten to twenty if you've surpassed ten printings, etc.), and the printer simply covers up the lower edition numbers when the book is being printed. Next time it's printed, the printer will cover up the next number.
Obscure! Bizarre! But it makes sense!
As for why different companies use different conventions for displaying the numbers, that is entirely due to the whims of the companies themselves, and it's because all the methods have different pros and cons. If they left-justify the numbers and print them from one to ten, it looks a little strange to have higher edition numbers floating off to the right. If they center them and alternate them (as in the example above), the number row always appears more-or-less centered on the page as the numbers are removed, but it's confusing to read. Some companies think it's best to have the numbers ascend from left to right (because it's the Western reading direction), while others prefer to ascend from right to left (more difficult to parse, but looks nicer).
If you'd like to learn more, check out this post and the accompanying comments. This information will never save your life, but maybe you've always wanted to know.