First Edition Books and The Number Line

I must admit when I first got back into collecting first edition books again I was not aware of the number line. It is possible that a few people reading this might be saying what's the number line?
I had always looked for the term "First Edition," "First Printing," "First Published," "Published," or "First Impression" to indicate whether a tome was a first.

Apparently now about 80% of publishers particularly in the US use the number line.

So what is it?

The number line is usually a series of numbers

9 8 7 6 5 4 3 2 1 or 8 6 4 2 1 3 5 7 9 etc

If a 1 is present in the line the book is a first edition first printing.

For a second printing the 1 is removed and the lowest number will be 2.

Pictured right is the number line from a first edition of Prey by Michael Crichton - this does not include any text mentioning First Published but does have 1 in the number line.

Some books may say First Published as well as having a number line.

I have several Eoin Colfer Artemis Fowl books. One of which I thought was a first edition

The first is Artemis Fowl with the following
First Published
10

indicating a First Edition 10th printing (so not really a first edition at all)

Occasionally the number line may be accompanied by a date line

9 8 7 6 5 4 3 90 91 92 indicates a third printing, published in 1992.

There seem to be a few exceptions to this rule.
Book Collecting Tips mentions that for Random House Publishers

A letter "B" in the middle of a number line that contains a "1" is an 11th edition.

Pictured right is a publisher page for Bridget Jones The Edge of Reason by Helen Fielding.
This includes the text First Published 1999 as well as a number line at the bottom. This indicates a First Edition First Printing.

I have noticed lots of price differences on certain books with different First Edition Printings.

As well as lots of books on ebay indicating First Editions. These may well be First Editions but not necessarily a First Printing.