If you look at a modern crochet hook, most likely the hook will have 3 things on it - other than the name of the company that manufactured it. These three things are a letter, a number, and a mm size.
These are the 3 common ways to designate hook sizes. Some companies use all 3 designations, some only one or two.
What can get confusing is that there is not a standard measurement for hooks. So one company's hook may have the same letter as another company's hook, but the mm size is different (i.e. an N in one company may be 10mm, in another it may be 9mm). That small difference may be just enough to alter your gauge. If a pattern calls for an N hook, and the designer used a 10mm N, but you use a 9mm N, your gauge may be off enough to really make a difference in the finished product. And you'll wonder why - you did use the hook that was called for. If the pattern doesn't list the mm size for the hook, it's really not your fault. If the mm size was listed along with the hook letter and the number, then you should make it a point to pick a hook based on the mm size. Crochet patterns should list mm sizes. Letters and numbers are fine for the hooks, but the mm size is the most important thing! If you remember that one little thing, you'll be way ahead!
One more thing to remember - some older hooks may have different mm sizes than newer ones from the same company. So, remember, the best thing to look for in a hook is the mm size.
Here are the commercially available size of hooks:
B-1, 2 mm or 2.5 mm
These are the commercially made hooks. There are other sizes, made by independent hook companies.
I hope this helps those who are confused by the variety of hook sizes and designations.