Blues Guitar Scales, Solos and Theory Lesson

Q:

In the songs Communication Breakdown, Whole Lotta Love and Good Times, Bad times I have found a little anomaly.  (This may happen in many Zeppelin Songs or throughout music I don't know).  All of these songs are in the key of A.  They have 3 #'s in the key signature, use AED chords...  they're in A.  The root in all of these songs is definitely E.  This would make the songs in Mixolydian Mode.  Even the key signature for Good Times Bad Times say Mixolydian mode.  All of the solos however in each of the songs use the  E minor scale (or G major).  Does that mean they change the modal sound to Dorian for the solos because there is no key change?  (G major scale over EAD, key of A)  Now don't get me wrong these songs and solos are awesome, but I am just trying to get some clarity on what is going on in the songs.  I remember you saying that if a song has a set modal feel already and you try to change it or force a differ net modal feel it will sound out of place.  Dorian and Aeolian are both bluesy sounds though and obviously sound good. It seems that they wanted a blues feel to the solo and put in a dorian mode solo over the mixolydian feel of the songs? (pretty sure that Mr. Page knew what he was doing but it has confused me a bit).  So my questions are what is going on here and if you have a different root than the major key can you just play either the major or minor scale of that root?

 

A: These songs follow the blues approach I discuss in Fretboard Theory Chapter 2 and then later again in Chapter 7. I also demonstrate this blues concept at the end of my DVD, Getting Started with the Pentatonic Scale.

Blues songs break away from traditional guitar theory by apply the minor pentatonic scale over a major based chord. Specifically, this occurs over a dominant seven chord (written as simply 7), or at least a chord functioning as a dominant seven. The only chord in a key that produces a dominant seventh is V (5). This is because the fifth scale degree is a major triad, and it has a flat 7th interval. As you stated correctly, Mixolydian Mode stems from this fifth degree.

Playing the minor pentatonic scale over a major based chord produces the follow intervals:

Root, b3, 4, 5, b7 (and possibly the b5 ala the "blues scale.")

So you nearly have all the intervals necessary to build a dominant seven chord, minus the major third. Playing a minor third over a major third produces some dissonance, but in a blues-based context this clash of notes creates an edgy sound that we've grown to like (it's rock 'n' roll, baby). Often times players will also add in the major third to minor pentatonic scale patterns.

So, blues-based rock songs can include Mixolydian Mode (a.k.a. the Dominant scale) and/or minor pentatonic. Oh, and you don't always have to break the rules. Blues-based players also use the major pentatonic scale over major chords as traditional guitar theory would teach.


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Pentatonic/CAGED/Progressions/Modes