I'm going through Garrison Fewell's "Jazz Improvisation for Guitar - A Harmonic Approach". The start of Chapter 1 reads:
Triad substitution is the use of triads derived from the chord tones and melodic extensions of a chord, beyond the basic triad. These triads can be categorized as either diatonic substitutions (triads that only contain chord degrees 1, 3, 5, 6, or 7) or upper-structure triads (triads that contain at least one tension)
So, in Cmaj7 Lydian we have:
Chord tones: C, E, G, B
Tensions: D, F#, A
Diatonic substitutions: Em, Am
Upper-structure triads: G, Bm, D
F#dim is an upper-structure triad but is unstable and not used as often
How does this translate to minor chords? Let's say I want to know the diatonic substitutions and upper-structure triads of Am7 in Dorian mode.
Chord tones: A, C, E, G
Tensions: B, D, F#
Now, C is one diatonic substitution, and the other seems to be F#dim (F# A C), but I don't hear it as "compatible" as the other diatonic subs (if that makes sense?), so I'm not sure about that one. The upper-structure triads would be Em, G, Bm, D.
Is this correct? For minor chords, the triad diatonic substitution and upper-structure triads are derived in the exact same way as in major chords? (And in a related note, in the Ionian and the other 6 modes derived from it, are there always two diatonic substitution triads?)