Robin,

I dunno exactly what you want to know, but a quick stop at Google yields, among other things this from the site Cyberfret.com:

Question #24
What is the Nashville numbering system?

Answer

The Nashville numbering system is a form of music notation where the chords are represented by numbers instead of letters. The number shows how a chord relates to the notes of a major scale of a particular key.

Here is a C major scale, and each note gets a number.

C D E F G A B
1 2 3 4 5 6 7

In the Nashville numbering system you might see something like this.

1///|4///|5///|1///

That translates to these chords.

C///|F///|G///|C///

You will use chord modifiers, like m, 7, sus2 etc to tell what type of chord will be played. So if you see something like this.....

1///|2m///|5///|1///|

.....it translates to this....

C///|Dm///|G///|C///

Here is a G major scale

G A B C D E F#
1 2 3 4 5 6 7

So in the Key of G.....

1///|2m///|5///|1///|

....would mean...

G///|Am///|D///|G///

The numbering system allows guitar players to understand how the chords relate to each other, yet not writing specific chords. This is helpful, especially when the key of a song may need to be changed to accommodate a vocalist in the studio on the spur of the moment. Or for using a capo.

If there is a root of a chord that is different from a note in the major scale, the b (flat) or # (sharp) symbol is used before the number.... b2m or b3 etc.

So if you see something like this for the key of C.....

1///|b3///|b7///|1///|

.... it would translate to.....

C///|Eb///|Bb///|C///

E would be 3, so Eb is b3. B would be 7, so Bb would be b7.

This system is really a lot like the numbering system used for traditional music theory. In music theory, you use Roman numerals (I, II, III, IV)...to serve the same purpose. So instead of seeing......

1///|2m///|5///|1///|

....you would see this.

I///|IIm///|V///|I///|

Now the people that read your Nashville numbering charts will need to know music theory in order to understand what chords to play.

Be sure to check out the lessons in the Theory Section here at Cyberfret.com.

etc. etc.

Doug-bob sez check it out.