Part 12: Blues, with the foundations still clear · Lesson 109 / 117
Blues harmony changes our diatonic baseline
Understand why A7–D7–E7 is not simply the A-major chord family.
In A major, the diatonic seventh chords on I, IV and V are Amaj7, Dmaj7 and E7. Our blues uses A7, D7 and E7 instead.
Inspect their notes: A7 = A–C#–E–G; D7 = D–F#–A–C; E7 = E–G#–B–D. G natural in A7 and C natural in D7 are outside A major. E7 also contains G#, so one fixed A-major note list does not describe the complete progression.
This is why we placed blues after diatonic harmony. The earlier rules were a clear reference, not a claim that all music uses only one scale. Here the established blues language gives dominant-seventh-quality chords different roles, including tonic A7.
Calling A7 “dominant seventh” names its quality. In this A blues it can function as I7, the tonic chord, rather than being treated automatically as V7 of D. Listen to the form and centre before assigning function.
Hear it
Blues Chords
Audio fileFour-beat count-in · 60 BPM · Warm sampled electric guitar. Listen once before playing along.
A concrete example
A7(I7), D7(IV7), E7(V7) in this A-centred blues. Their combined notes are not all members of A major.
Try it, slowly
- Name the third and seventh in each chord.
- Compare Amaj7 with A7 to hear G# versus G.
- Follow the form and identify A7 as the established tonic rather than renaming the key at every chord.
Check one idea
Why does the word dominant not force A7 to resolve to D in every passage?
Show the explanation
It can name the chord quality 1–3–5–b7. Its harmonic function depends on context; here A7 participates as the blues tonic.
A good place to stop
You can separate the blues treatment from the earlier diatonic seventh-chord family.
Return to the example after a short break. A clear explanation matters more than speed.