Chord Substitutions
Reharmonization is the art of swapping a plain chord for a richer one that does the same job. It's how the same melody can be dressed up a hundred ways. Here are the four workhorse substitutions — each one shown before and after so you can hear exactly what it does.
The Core Idea: same job, new colour
Every chord has a function (tonic, subdominant or dominant). A substitution replaces a chord with a different one that serves the same function — usually because they share notes or the same tension. The progression's logic stays intact; only the flavour changes.
Key
Relative / diatonic substitution
Before
After (substituted)
Chords a 3rd apart share two of their three notes, so they can stand in for each other. Here IV is replaced by its relative ii — same subdominant job, softer colour.
Secondary dominant
Before
After (substituted)
Turn a chord into the V7 of the next one to borrow that strong pull. Here a VI7 is inserted as the "V7 of ii" — it aims a spotlight at the ii chord.
Tritone substitution
Before
After (substituted)
Replace a V7 with the dominant a tritone away (♭II7). They share the same 3rd and 7th — the notes that define the pull — but the bass slides chromatically down into the tonic.
Diminished passing chord
Before
After (substituted)
Bridge two diatonic chords a whole step apart with a diminished chord a semitone between them. It adds smooth chromatic voice leading and a touch of tension.
How to choose a substitution
A substitute must still contain (or support) the melody note on that beat. That's the first filter.
Swap tonic for tonic, dominant for dominant. Shared notes (a 3rd away) or a shared tritone (tritone sub) keep the function intact.
Secondary dominants and passing diminished chords insert extra pull between chords — great for filling static bars.
Reharmonize your own tunes
Try these swaps in Song Lab, or connect them to the ii–V–I and modal interchange.