Ninths and Thirteenths

Extensions are notes stacked above the seventh — the ninth, eleventh and thirteenth. They change the colour of a chord without changing its function, which means you can add them wherever you like and the harmony still works.

That last point is worth dwelling on. If a chart says Gm7, you can play Gm9 and nothing breaks. The chord is still the ii of F, still minor, still in the same place in the form. It just sounds more sophisticated. Most of what separates a jazz guitarist's comping from a beginner's is substitutions this small, applied consistently.

A rule of thumb for where they work: on minor sevenths and major sevenths, ninths sound good almost always. On dominants, ninths and thirteenths sound smooth while flattened ninths sound tense — so use the tense ones where the music is about to resolve, as you did in the jazz blues.

A warning worth taking seriously: extensions crowd fast. Four notes is plenty. Drop the root — the bass player has it — and use the space for the ninth instead.

Gm9

Gm9. Play it anywhere the chart calls for Gm7 and listen to what the extra note does.

Cm9

Cm9 — the identical grip seven frets lower, on the other minor chord in the track below.

This track sits in G minor at 90 BPM and its changes lean on Gm7, Cm7, Am7 and Dm7 — a lot of minor sevenths, which makes it ideal for practising this one substitution.

Play one full pass using only plain sevenths. Then play a second pass substituting a ninth every time, changing nothing else.

The difference between those two passes is a large part of what people mean when they say a player sounds jazzy.

G minor at 90 BPM. One pass plain, one pass with ninths. Nothing else changes.