To understand the Nashville Number System, it's important to first understand the likely chords to appear in a major key. "Diatonic" means within the key. Forming triads from each scale tone's root note will result in the following chords:
Major (1), Minor (2-), Minor (3-), Major (4), Major (5), Minor (6), min7b5 (a.k.a. half-diminished seventh) (7m7b5)
In Nashville Number Charts, "-" is used to denote minor. Let's examine all of the diatonic chords, starting from each scale tone, in the key of G.
G Major
1. G Major is the first chord in the key of G Major.