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<h1>Chord Substitutions</h1>
<p>Chord substitution is replacing one chord in a progression with another that serves a similar harmonic function. This works either because the two chords share important notes, or because they create similar tension and resolution patterns. Substitutions are how jazz musicians personalise the changes of a standard and how composers create harmonic interest without abandoning the form. The same tune can sound completely fresh with well-chosen substitutions.</p>

<h2>Tritone Substitution</h2>
<p>The most important substitution in jazz. Any dominant seventh chord can be replaced by the dominant seventh chord a tritone (six half steps) away. The reason it works: both chords share the same tritone between their 3rd and 7th, just with those roles swapped:</p>
<pre>
G7 tritone sub = D♭7

G7:  G — B — D — F      3rd=B,  7th=F   → tritone B↔F
D♭7: D♭ — F — A♭ — C♭  3rd=F,  7th=C♭  → tritone F↔C♭(=B)

Both contain B and F — in G7 they are 3rd and 7th;
in D♭7 they are 7th and 3rd (reversed).

ii-V-I in C:
Original:    Dm7 → G7  → Cmaj7
Tritone sub: Dm7 → D♭7 → Cmaj7

D♭7 → Cmaj7: bass moves by descending half step (stronger than G→C fifth)
</pre>

<table>
  <thead><tr><th>Original V7</th><th>Tritone sub</th><th>In the key of</th><th>Bass movement to I</th></tr></thead>
  <tbody>
    <tr><td>G7</td><td>D♭7</td><td>C</td><td>D♭ → C (half step down)</td></tr>
    <tr><td>C7</td><td>F#7</td><td>F</td><td>F# → F (half step down)</td></tr>
    <tr><td>F7</td><td>B7</td><td>B♭</td><td>B → B♭ (half step down)</td></tr>
    <tr><td>B♭7</td><td>E7</td><td>E♭</td><td>E → E♭ (half step down)</td></tr>
    <tr><td>E♭7</td><td>A7</td><td>A♭</td><td>A → A♭ (half step down)</td></tr>
    <tr><td>A♭7</td><td>D7</td><td>D♭</td><td>D → D♭ (half step down)</td></tr>
  </tbody>
</table>

<h2>Diatonic Substitution</h2>
<p>Chords that share most of their notes can substitute for each other, creating a softer or alternative version of the same harmonic function:</p>

<table>
  <thead><tr><th>Original chord</th><th>Substitute</th><th>Shared notes (C major)</th><th>Effect</th></tr></thead>
  <tbody>
    <tr><td>Cmaj7 (I)</td><td>Em7 (iii)</td><td>E, G, B</td><td>Softer tonic — iii sounds slightly less resolved than I</td></tr>
    <tr><td>Cmaj7 (I)</td><td>Am7 (vi)</td><td>C, E, G</td><td>Minor colour for a major tonic — darker</td></tr>
    <tr><td>Fmaj7 (IV)</td><td>Dm7 (ii)</td><td>F, A, C</td><td>Pre-dominant function either way; Dm7 has stronger pull to V</td></tr>
    <tr><td>G7 (V)</td><td>Bm7♭5 (vii°)</td><td>B, D, F</td><td>Both dominant function; vii° more ambiguous</td></tr>
  </tbody>
</table>

<h2>Relative Major/Minor Substitution</h2>
<pre>
Major chord ↔ its relative minor (built on major's 6th degree):
  Cmaj7 ↔ Am7     (Am7 is vi of C major)
  Fmaj7 ↔ Dm7
  B♭maj7 ↔ Gm7

Minor chord ↔ its relative major:
  Am7 ↔ Cmaj7
  Dm7 ↔ Fmaj7

Works in both directions for reharmonisation.
Substituting Am7 for Cmaj7 gives a minor tonic colour when the original is major.
Substituting Cmaj7 for Am7 gives a major lift to a minor chord.
</pre>

<h2>Secondary Dominants</h2>
<p>Any diatonic chord can be preceded by its own dominant seventh chord — temporarily tonicising the target chord. The inserted dominant is "borrowed" from outside the key:</p>
<pre>
V7/ii (dominant of ii): A7 → Dm7
  A7 is outside C major but resolves strongly into Dm7.
  Creates a momentary feeling that Dm is the tonic.

V7/IV: C7 → Fmaj7
V7/V (most common): D7 → G7 → Cmaj7
V7/vi: E7 → Am7

Standard: ... Cmaj7 — Dm7 ...
With sec. dominant: ... Cmaj7 — A7 — Dm7 ...

The A7 prepares Dm7 with a proper V→I motion inside the progression.
</pre>

<h2>Diminished Seventh Substitution</h2>
<p>A diminished seventh chord can substitute for the dominant seventh chord a half step below its root. They share three of four notes — the diminished seventh is essentially a rootless dominant 7♭9:</p>
<pre>
G7♭9: G  B  D  F  A♭
Bdim7:   B  D  F  A♭   (= G7♭9 without the root G)

Bdim7 substitutes for G7 (or G7♭9), especially for inner voice movement
and passing chord use. The diminished chord is more ambiguous than a G7
because it has no clear root on its own.

Diminished seventh is symmetrically built (all minor thirds):
  Bdim7 = B D F A♭
  Ddim7 = D F A♭ B   (same four notes, different inversion)
  Fdim7 = F A♭ B D   (same four notes again)
  A♭dim7 = A♭ B D F  (same again)

So Bdim7, Ddim7, Fdim7, and A♭dim7 are all the same chord with different roots.
This means one diminished seventh chord can function as a sub for four different dominants.
</pre>

<h2>Passing Chords</h2>
<p>Insert a chord that connects two existing chords by chromatic or stepwise bass motion:</p>
<pre>
Original:  Cmaj7 — — — Am7
With pass: Cmaj7 — C#dim7 — Dm7 — — Am7
           (bass: C → C# → D — chromatic)

Or descending:
Original:  Cmaj7 — Am7
With pass: Cmaj7 — G/B — Am7   (bass: C → B → A — stepwise)

Passing chords don't need to have a specific harmonic function —
their job is smooth bass movement between harmonic destinations.
</pre>

<h2>Reharmonisation for Chord Melody</h2>
<p>These substitutions are the tools of reharmonisation — changing chords under a fixed melody. The only rule: the melody note must work over the substitute chord (as a chord tone or an acceptable extension). A G over a Cmaj7 (G = perfect 5th, fine). A G over a D♭7 (G = major 3rd of D♭7, fine — tritone sub works). A G over an E♭maj7 (G = major 3rd, fine). Almost anything can work if the voice leading is smooth and the melody note is a chord tone or usable extension.</p>

<h2>References</h2>
<ul>
  <li>Levine — <em>The Jazz Theory Book</em></li>
  <li>Rawlins &amp; Bahha — <em>Jazzology</em></li>
  <li>Coker — <em>Improvising Jazz</em></li>
  <li><a href="https://www.jazzguitar.be/blog/chord-substitutions/" target="_blank" rel="noopener">jazzguitar.be — Chord Substitutions ↗</a></li>
</ul>
