The beauty of the Nashville Number System is that someone can indicate the key at the beginning of a tune, or even signal a key change in the middle of a tune, by calling out the key or by holding up a number of fingers, and as if by magic, everyone is in step and in key.
Although the key has changed, and the actual chords have changed, the chord numbers remain the same. It requires some memorization and a little math in your head, on the fly. I understand the concept but I can’t do it. I’m told it gets easy after using it for a while.
I witnessed it in action several times today during a jam I attended. They’d call out keys and use the numbers mostly to figure out which chords to use; sometimes before or after the song, they’d discuss it and refer to particular chords by numbers, not the chord names. They’d start a song and the singer realized it wasn’t in his range, so he’d yell out a new key and everyone just went along and it sounded great. A bunch of freaks I tell ya. Music nerds. Yeah, one day, that’ll be me.
Just FYI, these guys were playing mostly in the keys of G, A, Bflat, and C. Once in D I think.