What Do The Numbers Mean

To dilute the info provided above,

There are widely accepted Proper grades in the industry.

G - Good
VG - Very Good
MS - Mint State
(many more)

Etc. Also, there are often times coins that fit in between these grades. For instance you have a coin that is certainly not VG, because of an amount of wear in a key area, yet it is way to Nice to just call it a plain ole Good.

This is where the numbers come in. They allow us to differentiate varying grades within a grade.

An exaggerated example is in the AU Grade. You can have;

AU50 - Not quite enough wear to be an Extra Fine, but obvious.
AU53 - Wear is apparent, but not distracting and overly noticeable
AU55 - Slight traces of wear are visible on a few places, but take some effort to see.
AU58 - Wear is only noticeable under magnification and on the highest points of the coin.

Past this you are in MS (Mint state) territory, and its varying levels.

There is a whole school of thought on placing a grade on a coin between MS64 - MS 70, so I wont go into it.

Hope this helps.

Welcome to that confusion of a numbering system. Way back we only had G, F, Unc and Proof. And it all worked out great. Then naturally someone said well this is better than G but not as good as F so I'll call it VG or AF for Very Good or Almost Fine. This got more and more of those people with nothing to do to continue this with Almost Good, Almost fine, Almost anything. Next thing everyone was joining in with AG, G, AF, F, VF and on and on and on.
Along the way someone said this could be done with numbers so everyone wouldn't have to know that G is Good. So there was this 1 to 70 point system started but still many wanted the G, F, etc. so all these silly numbers ended up with the letters attached. Which is why we now have VG-8, AU-50, F-12 and all the rest.
Oddly enough if you look through the famous Red Book, 2013 edition, this is sort of explained on page 9. This to some would indicate that with that so very carefully explained, all throughout this book, that system would be used. Right?
Well then check out page 87?

No, not really. Each letter grade (like G or VG) has a baseline, and everything below that baseline must be a lower letter grade. You saw this in the explanation of AU. For instance, if you thought you had almost an AU-50, but "not quite," you could not say AU-49, as the 40s grades are in the realm of Extremely Fine. I know it's confusing.

VG should start at 8 as in VG-8.

Tell you what, I have a couple of books on grading that I can send you free of charge. Once you get 50 posts, good posts, just PM me and I'll send you one that will lay out the system of how you might grade a coin.

Keep in mind that it takes a good amount of experience before your grading skills will allow you to recognize a coin's true condition. Hang in there and keep looking at coins. Oh, and when someone asks for an opinion as to a coins grade in the forum, you might want to participate. Just walk in and test the water. Everyone on the forum helped me.

Andoyan 62,great question.
Very informative answers but, why does "Grading Coins by Photographs" by Q. David Bowers still show coins graded AG-3 ( About Good)?
Is the AG grade out.

Not Quite.

The Letter grade and number grade are two separate grading systems. We just pin the two together for Correlation and consistency.

So in the case of AU, the number grade 50 - 58 would be the same as saying AU, or 20 - 35 would be VF.

One system has precise standards for each grade (Numerical)
The other is approximate, at a glace, for each grade (Letter)

....there is no AU60 or 61. Actually there is. Its AU coins graded MS by TPGs that have lowered their standards.