Questions: Jazz Harmony Basics

Question 1

In the key of C, what is the tritone substitution for G7?

• A. Ab7
• B. Db7
• C. F7
• D. Bb7

Answer: (B)

A tritone is six semitones. Six semitones above G is Db (G–Ab–A–Bb–B–C–Db). So the tritone substitution for G7 is Db7. The key insight is that G7 and Db7 share the same tritone internally: G7 contains B and F (a tritone apart), and Db7 also contains F (=E#) and Cb (=B) — the same two pitches, just spelled differently. This shared tritone is what makes the substitution harmonically coherent.

Question 2

The ii–V–I progression in jazz is harmonically unrelated to the authentic cadence in classical harmony.

• T. True
• F. False

Answer: False

Jazz harmony is an extension of the same tonal system that underlies classical harmony, not a separate language. The ii–V–I is the authentic cadence (V–I, typically preceded by ii or IV) with seventh chords added throughout. The pull of the V7 to I is identical in both traditions. What jazz adds is richer chord extensions, alterations, and substitutions built on top of the same functional foundation.

Question 3

Why does tritone substitution create smooth chromatic bass motion when resolving to the tonic?

Model answer: The substitute dominant is rooted a tritone away from the original dominant — in C major, Db7 instead of G7. Db is a half-step above the tonic C, so the bass resolves downward by a semitone (Db → C) rather than the whole step descent (G is a fifth above, but the bass ascends a fourth or descends a fifth). This half-step approach is smoother and more chromatic than the original root motion.

The original V–I motion (G to C) involves a descending fifth or ascending fourth in the bass — a strong but somewhat angular move. The tritone sub (Db to C) replaces this with a descending half-step, the smoothest possible bass motion. This is why tritone substitutions became standard in bebop: they connect chord roots by half-step, creating a more chromatic and sinuous bass line.