Critical intervals

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Continuing our discussion on the subject of intervals. There are two groups of intervals that seem to be more critical in their effect than the others - these are the 3rd and 7th intervals.

The 3rd interval determines the overall tonality of a key, scale, chord or phrase: major third produces major tonality a minor 3rd produces minor tonality.

If you combine both tonalities you get a very disturbing sound. An excellent example of this is the 7#9 chord which features strongly in many of Jimi Hendrix' classic songs. The formula for this chord is 1 3 5 b7 #9 The major 3rd is obvious enough, but the minor 3rd is disguised as #9(Remember the 9th note is the same as the 2nd and if you sharpen a 2nd you'll get the same note as if you'd flatted a 3rd!)

Blues and Rock 'n' Roll licks rely heavily for their effect on using the transition from minor to major 3rd. Check out this simple blues lick in G; if you were playing against a G major chord you would play this:

E---------------3-----------------------
B----------------------------------------
G-----3-h-4------3-h-4-p-3-h-4----
D----------------------------------------
A----------------------------------------
E----------------------------------------

i.e the final note is a B (major 3rd in the key of G)

But if you were playing this lick against a Gm chord you would make sure you ended the phrase on the Bb (minor 3rd in the key of G):
E------------3---------------------------
B-----------------------------------------
G--3-h-4------3-h-4-p-3-h-4-p-3--
D-----------------------------------------
A-----------------------------------------
E-----------------------------------------

Try it out!

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