— Begin quote from "beardedbanjo"
Regarding the Nashville Number System:
Ben has Old Joe Clark in the key of G. The chords and corresponding numbers would be:
G - I
D - V
F - ?? (F# would be the VII chord, yes?)
So how do you number an F chord when in the key of G?
— End quote
Great Question and good observation!
Although Fiddlewood is exactly right F is the flatted 7th note, (used a lot in blues, rock, & bluegrass) I think you were talking chords and you would not refer to the F Major chord here as a flatted 7th since the 7 chord is actually a diminished/flat 7, and flatting that is way off the subject of this thread.
SHORT ANSWER: Old Joe Clark, like some other old fiddle tunes,(like June Apple) is Modal (mixolydian to be exact) or references an older system of music, which is still with us, that predates the current system of major minor , and modern chords (7ths, flat fives, diminished etc.) So the Nashville numbering system won’t quite work perfectly on some old time tunes.
LONG ANSWER: SO …. Welcome to the Modes! The good news is it is not rocket science. In this case OJC is based on the mixolydian mode or G to G in the C scale (which has the flat 7 F note BTW). So what the heck is a Mode. Well before people figured out to classified major and minor, they would break up each major scale into 7 sub scales, each part starting on a different note to get different feels. So C to C in the C major scale CDEFGABC is Ionian but D to D in the C major scale is dorian, E to E in the C scale is phrygian etc… (see the chart at the bottom). BTW Dorian is used a lot in bluegrass.
So since we call Old Joe Clark “G” and it pretends to be in “G major” for the first part of the song with the D chord (the Keys of C and G only have a one note difference F and F#) it switches to G Mixolydian on the F chord or the IV of the C scale.
Fun fact: Ionian is also called the Major scale, but Aeolian is called the Natural Minor Scale. So A minor and C major are really the same scale, they just start on different notes. So when people say what is the relative major or minor scale this is what they mean. What is relative minor scale to G major? E minor …same notes . Congrats you kind a know twice as many scales as you thought.
Mode Tonic relative to major scale Interval sequence (T=whole tone or full step; s=semi tone or half step). This works with other Keys- just using C as and example.
Example
Ionian I : T-T-s-T-T-T-s : C-D-E-F-G-A-B-C
Dorian II : T-s-T-T-T-s-T : D-E-F-G-A-B-C-D
Phrygian III : s-T-T-T-s-T-T : E-F-G-A-B-C-D-E
Lydian IV : T-T-T-s-T-T-s : F-G-A-B-C-D-E-F
Mixolydian V : T-T-s-T-T-s-T : G-A-B-C-D-E-F-G
Aeolian VI : T-s-T-T-s-T-T : A-B-C-D-E-F-G-A
Locrian VII : s-T-T-s-T-T-T : B-C-D-E-F-G-A-B