The Nashville Number System, Also Called the Nashville Chord System

The Nashville Number System (or Nashville Chord System) works like this:

Each chord gets notated on the lead sheet according to the scale number of the chord's root, regardless of the key. For example, the chord built on the tonic note of the scale is always notated as the number 1; the chord built on the fourth note of the scale is always notated as the number 4; and so on.
You can (and should) use Roman numerals to represent the chords, but in Nashville they usually use Arabic numbers (which can be confusing, since Arabic numbers apply to scale notes as well).
For the minor triads, add a lower case “m” to the number.
For the diminished chord, add the symbol “º”. Add other symbols as needed for different extensions of chords such as ninths.