[ I first wrote on this over a year ago. I've since gotten some additional insight from my tech writer perspective.]
My first Dobro lesson was with Mark Van Allen who spent two hours reviewing theory with me. It was a fascinating tour of the fret board, scales, and chords. Mark pointed out that if you play the scale of G on all the strings, and if you put a dot down every place you "fretted" a string, you create the basic chords included in the key of G.
G B D 0 0 0 GBD G major 2 1 2 ACE A minor 4 3 4 BDF# B minor 5 5 5 CEG C major 7 7 7 DF#A D major 9 8 9 EGB E minor 11 10 10 F#AC F# dim
Mark describes the patterns as bar, chevron, chevron, bar, bar, chevron, hockey stick (oh yeah, we'll talk more about that hockey stick later).
Recently, I was at the Steve Kaufman Acoustic Kamp where Ivan Rosenberg established the same set of patterns. He did it by applying the following technique for making a triad chord: "Play a note, skip a note, play a note, skip a note, play a note." He started at G and went up the scale, producing the same pattern that Mark had.
The first six chords have a particular beauty and closure about them. The three major chords are the tonic (G) dominant (D) and subdominant (C) chords. The three minor chords are their respective relative minors. (A relative minor is the minor that has the same key signature as its relative major. Their scales share the same notes but start on different roots-one yielding a major scale and one yielding a minor scale.) So for G, we have E min, for C we have A min, and for D we have B min.
Then we get that darn F# diminished! What's with that? It's like meeting the folks in a jam: The banjo guy says, "I'm Fred," the mandolin player says, "I'm Skip," the guitar player says, "I'm Jack," and then the fiddler says, "I'm Throckmorton." What the...??!! How did he end up with the others?
Even Ivan kind of dismissed the F# diminished saying it wasn't very useful for what he wanted to cover that day. The problem I have with it is that it just doesn't seem to belong. Have you EVER played a song in G where an F# diminished showed up? I haven't. But the other six chords are mainstays for G progressions.
I've fretted (no pun intended) over this for a while, and I think I've resolved it, at least for me. That seventh chord is a D7 without the root D. A seventh is the 1 3 5 b7 and in the case of D the 3, 5, and flatted 7 would be F#, A, C--exactly what we have. That would make more sense, except now I have the problem of where did the lost root go? That's the problem I have been pondering for the last year or so.
Then my tech writer background kicked in and solved it. Think of these seven chords as a seven-page document in a word processor. The first six are in Portrait and the page is three strings wide. That's plenty wide enough because major and minor chords are properly rendered with three notes. 1, 3, 5 or a 1, b3, 5.
But what if the last page is supposed to be in Landscape? That would make the page four strings wide and--lo and behold--look what we now see if we go into Landscape view: D, F#, A, C our D7. We were missing the root because it takes four notes to properly render a dominant 7th and we were looking at just the last three! Now it's as if that fiddler named Throckmorton winks at me and adds, "But my friends call me Buddy." The world makes sense again.
OK, one problem solved, but I now have two more: (1) Why doesn't F# get its own chord and (2) why does D get two chords?
Once again my tech writer perspective kicks in on the first one. In document design or screen layout, there is a well-known phenomenon that if two elements are too close they create a visual tension. For example, text too close to table borders are the visual equivalent of nails on a chalk board. Or put a button too close to the edge of the screen and the user begs for some padding to give it some breathing room. I think F# is just too close to G! Our ears don't like a chord that is only a half-step away from the root.
And as far as D getting two chords when everybody else gets just one? Well, maybe that's why it's called the dominant.