Answer · Theory
What is a ii-V-I, and why is it used so much?
A ii-V-I is three chords built on the second, fifth and first notes of a key: in C major, Dm7, G7 and Cmaj7. Each root falls a fifth to the next, and the dominant chord's tension resolves by step into the tonic. No other three chords establish a key as clearly, which is why it is the basic unit of jazz harmony and common everywhere else.
What each chord does
- ii (Dm7) prepares. It leads away from home without much tension.
- V (G7) is the dominant. It holds B and F, a tritone apart, which is the least stable interval in the key.
- I (Cmaj7) is the tonic. B rises a semitone to C and F falls a semitone to E, and the tension is resolved.
Preparation, tension, resolution: the progression is a complete sentence in three chords.
Why it sounds smooth
The roots move down in fifths (D to G to C), the strongest root movement there is. Above them, the notes that define each chord move by a single step. The seventh of Dm7 (C) falls to the third of G7 (B). The seventh of G7 (F) falls to the third of Cmaj7 (E). A pianist or guitarist can play the whole thing while moving two fingers one fret or one key each.
In a minor key
The minor version is iiø7, V7, i: in C minor, Dm7♭5, G7, Cm. The V chord is still major, borrowed from the harmonic minor scale, so the pull to the tonic is as strong.
Where you hear it
- Jazz standards are largely chains of ii-V-Is, each one pointing at a new temporary key. A ii-V that never reaches its I is a standard way to change key.
- Pop, soul and R&B use it at the end of a phrase, or replace the ii with IV (F, G, C), which does the same preparing job.
- Lo-fi and neo-soul lean on the seventh-chord version for its colour.
In Zammoth
The engineer writes harmony as MIDI in the key you name, so a progression can be asked for in the same breath as the tempo and the instruments:
The result is ordinary MIDI clips. Open one in the piano roll to change a voicing, and switch SCALE on to keep new notes inside the key. See the manual on the piano roll.
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