If you've always wanted to know how the Nashville Number System worked,
here is a link to a Primier Guitar page that explains it.
When I played harp in the Nashville studios I would bring my steno pad and
write numbers in it as the song was being demonstrated. My reasons for
doing so had little to do with why the other cats were doing it.
That is, it wasn't very useful for mastering the extremely simple parts
that I had to come up with on harp. After a single hearing I knew the
song. I wasn't going to be playing diminished or suspended notes, I was
there to create some pretty fills and solos, and occasionally sneak in some
rhythm work.
But if everyone else was writing stuff down during the demo, I felt that I
looked like a bozo if I wasn't writing something too. That would have been
the single most important reason I wrote my numbers.
Second most important was that it wasn't such a bad idea to get a visual
picture of the chord structure. It wasn't all that useful, but it was one
less thing to think about.
Finally, if Diamonds were agreed upon, I'd add them onto my sheet. It gave
me a reason to actually read the numbers, so as to not miss the Diamonds --
which would have given everyone the mistaken impression that I was the bozo
I'd cleverly avoided seeming like by writing numbers onto my pad back when
everyone else did.
Originally, one of the guys in the Jordanaires came up with the number
system for use by the Jordanaires. Some non-Jordinaires realized that it'd
be useful if, after writing out the chords, it turned out that the singer
needed a key change. No time would need be wasted writing the
transposition. Most of the Nashville guys I knew were skilled, trained
musicians who could transpose in their heads in real time.
I could transpose as fast as I could switch harps. You can too if you're a
diatonic player.
However, if you're a chromaticist and you can improvise around chords and
the music stays in the same key and doesn't modulate, and the producer
wants you to improvise all over the place, the number system becomes highly
useful. (Donna Lee doesn't sit well in the Nashville number system.)
And if you're hired to play one of those instruments that was considered a
real instrument by the American Federation of Musicians before 1949, like
say guitars, pianos, basses, etc, the number system is positively golden.