The Nashville Number System uses the roman numerals to identify chords and chord progressions in a particular song. This is very useful if a bandleader want’s to change the key of a song without having to re-write an entire chart.
The key may be specified at the top of the chart, or just decided verbally before the start of the performance or recording session. The numbers do not change when transposing the composition into another key as they are simply relative to the new root note.
A very useful system, however you will need to know the major scale for the given key. My first thought was wow there are twelve keys to know, how am I going to remember all that?
After years of teaching and using this system I realised the best way to learn this stuff is to get deep with one key first. Do the work now and you will have that benchmark key of C to refer back too when you move to another key.
Learn the fundamentals now ! Trust me this stuff is worth learning no matter what style of music you want to play
In the example below the roman numerals are placed under the chord- lower case red are the minor chords – higher case blue are the major chords