The Nashville Number System (NNS) uses the formula of the diatonic harmonies. It is a shorthand system which does not follow exact textbook theory practices, but is brilliantly practical for it's simplicity and it is applied all over the world in recording centers for that reason.
The Nashville Number system is always "All Majors". The NNS follows the formula of the major scale's diatonic interval/distances and the pattern of numbers is completely void of minors. Don't apply diatonic theory concepts such as figured bass or 1 2m 3m 4 5 6m 7dim... that will screw with the concept. In the NNS system, the numbers are always assumed to be major chords unless noted as different.
INTERVALS
These are the Major Scale Interval distances from the Root:
1 tone, 1 tone, 1/2 tone, 1 tone, 1 tone, 1 tone, 1/2 tone. You may see this discussed as W-W-H-W-W-W-H, where "W" is a whole-tone and "H" is a half-tone.
The NNS requires a number for each tone. I learned it by remembering these distances as "2 tones, 1/2 tone, 3 tones, 1/2 tone", knowing each tonal step requires a unique letter name. The NNS gives each tone a numeral instead.
C = the root or 1 chord D = "2" chord - it is found 1 tone (2 frets) up from C E = "3" chord - it is also 1 tone (2 frets) up from D F = "4" chord - it is found 1/2 tone (1 fret) up from E G = "5" chord - it is found 1 tone (2 frets) up from F A = "6" chord - it is found 1 tone (2 frets) up from G B = "7" chord - it is found 1 tone (2 frets) up from A C = is the start of the 1 chord's infinite repeatable pattern and is 1/2 tone (1 fret) up from B.
CHORD QUALITY
All chords are considered to be Major - No Minors for the NNS - until the musical mathematics are sorted. Every chord following this formula can be named with a number on a chart (using the NNS). The quality of the chord: major, minor, diminished, augmented, and all of the extensions above the number including altered intervals can be added.
KEY CENTERS NNS chart key centers can change depending on who writes the changes, according to the chart writers interpretation of what they hear as the songs key center. I recently was on a session where the chart writer was asked to rewrite the chart to make it easier for the band to read and therefore interpret the song.
He heard the first chord of the intro as a 1 chord. The band voted to have it written as a 4 chord, making the numbers in "C" read 4 1 6m 1. He wrote it as 1 5 3m 5, which changed the key center to "F' instead of "C". The numbers give the exact chords as long as the key center of the chart is known and agreed upon by all reading it.
These are the rest of the NNS numbers in the key of C:
The C# position = 1# or 2b also known as the Db chord The D# position = 2# or 3b also known as the Eb chord The F# position = 4# or 5b also known as the Gb chord The G# position = 5# or 6b also known as the Ab chord The A# position = 6# or 7b also known as the Bb chord
That completes the system...
EXCEPTIONS AND TRADITIONS
A chromatic rule that is followed (but many times broken depending on who writes the chart) for which numeral to choose out of these when there are two options as the chromatic positions present. When descending back into any of these chromatic positions...many chart writers use the "b" or "flat" numeral. Ascending up many chart writers use the "#" or "sharp" numeral.
There are two typical numbers that chart writers always use which breaks the ascending and descending rule - The 3b ("three flat") and the 7b ("seven flat") are almost always written instead of the 2# and 6#. They are always chosen because it eliminates one less step for the reader to transpose when they actually are the same chord. So the 3b is always a 3b and the 7b is always a 7b. These chords happen so often in song compositions it just simplifies the NNS.
Honestly I can not remember ever seeing a 6# or 2# for a major chord played in those positions. I point this out because on sessions I have seen charts written from all musical education and pro experience levels. It doesn't matter if session number charts composed by Pat Coil (North Texas State prodigy) to Charlie McCoy (earlier pioneer of the NNS) the 3b / 7b holds as the true choice.
SIMPLE OR COMPLEX
The NNS can get as complex as the writer of the chart wants to make it or it can be oversimplified. Music changes, notation changes with it.
In C an A = 6
A minor = 6m or 6- which is the more common way to notate the chord when it is minor
Ab9 = 6b9
So a progression in "C" going from C to F to Fm to Em7 to Am to Dm7 to G7/9 to C written in Nashville Numbers looks like this:
So again think of numbers on a NNS as "Always Being Major" until the writer of the chart designates which alteration is being played. This allows simplification and prevents arguments for those who live in theory weeds. The point of the system is to have a chart that always interprets the same no matter what key the singer wants to switch to and most importantly never takes any outside knowledge from the system and inserts it into the chart. Letter names require transposing all of those chords to new letters whereas numbers never change. The NNS is brilliant and saves time = $$$'s saved on production costs is why this system is used globally.