Triad over Root Substitutions
You have already learned that chords can have more than one name. This is valuable to know when it comes to chord substitution. Notice that an E minor triad contains the notes E, G and B. A Cmaj7 chord contains the notes C, E, G and B. Therefore, a Cmaj7 chord can be implied by playing an E minor triad over a C root note. This is called triad over root substitution.
On of the most common forms of this type of substitution is to use triads from within the same key as the original chord. In any key you can always substitute the chord two degrees ahead in the scale, thus creating a higher extension of the chord. This is shown here in the key of C major.
The same principle applies to modes. The next example shows a series of triads from the key of D major played over an A bass note. Since A is the 5th degree of the key of D, this suggests a mixolydian tonality. Mixolydian based triad over root substitution is useful in a Blues context, as each of the triads can suggest upper extensions of dominant chords.
The following Blues in the key of A uses triad over root substitutions to suggest various higher extensions of dominant chords built on A, D and E which are I, IV and V in this key. This is a way of creating interesting parts over one chord symbol.
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