The major ii–V–I and guide tones

If jazz had a password, it would be ii–V–I (“two-five-one”). In C: Dm7 – G7 – Cmaj7. Predominant, dominant, tonic, with a bass falling in fifths. Jazz standards are woven from it: you'll find ii–V–Is in every key, sometimes one per bar. Being able to play it instantly in all twelve keys is goal number one for any jazz pianist.

Why seventh chords?

In jazz, a bare triad sounds “unfinished”. The basic unit is the seventh chord (often enriched with 9ths, 11ths, 13ths). Each chord of the ii–V–I has its characteristic seventh: minor on ii, dominant on V, major on I.

Guide tones

The two notes that define a seventh chord are its third and its seventh: the guide tones. In a ii–V–I they form two lines that barely move:

• the 7th of Dm7 (C) falls a semitone to the 3rd of G7 (B), which stays put to become the 7th of Cmaj7 (B);
• the 3rd of Dm7 (F) stays put to become the 7th of G7 (F), which falls to the 3rd of Cmaj7 (E).

M:4/4
L:1/4
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:C
[V:RH] "Dm7"[Fc]4 | "G7"[FB]4 | "Cmaj7"[EB]4 |]
[V:LH] D,4 | G,,4 | C,4 |]

This is the backbone of all jazz harmony: with the bass and these two notes, you already hear the progression perfectly.

Associated scales

For improvising, each chord of the major ii–V–I pairs with a mode of the C major scale: Dorian over Dm7, Mixolydian over G7, Ionian over Cmaj7. They're the same seven notes! The difference lies in which notes you emphasise: guide tones and chord tones on strong beats.

M:4/4
L:1/8
Q:1/4=120
K:C
"Dm7"D F A c e d c A | "G7"B d f a g f d B | "Cmaj7"c8 |]

This line arpeggiates Dm7 up to the 9th, then G7 up to the 9th starting from the third, B, which resolves naturally to C. Play it with a swing feel: eighth notes in long-short pairs, like a triplet rhythm.