Most jazz improvisation methods tell you which scale to play over which chord — but that chord-by-chord approach leaves out the most important question: why is that chord there, and what does its position in the key tell you about which extensions actually sound right? This masterclass with saxophonist Joel Purnell cuts straight to the heart of that problem.
What you’ll get out of this lesson
You’ll learn a clear, context-driven method for working out the correct extensions for any chord in a standard jazz progression. By the end you’ll understand why the same chord type can need different extensions depending on where it sits in the key, and you’ll have a practical framework for working this out yourself — no expensive textbooks required.
What extensions actually are
An extension is simply one of the notes that sit between the chord tones — the 9th, 11th, and 13th (or their altered versions). Those are the notes that, when added to a chord, give you the scale. The chord tones — root, third, fifth, seventh — will nearly always sound fine regardless of context. It is the extensions where the real colour lives, and also where a lot of improvisers go wrong by defaulting to a textbook scale without checking the harmonic context first.
Why context matters more than chord type
The standard approach of playing mixolydian over any dominant 7th chord works roughly because the chord tones are always right. But the extensions that mixolydian implies may be entirely wrong for how a particular dominant chord is functioning in its progression. An F7 in the key of F is a completely different harmonic situation from an F7 in the key of B flat, or an F7 used as a tritone substitution. The method Joel outlines focuses on standard repertoire — the American songbook, Cole Porter, Gershwin, the music Sinatra sang — because that is where this kind of functional harmony is most consistent and most important to get right.
Everyone just plays mixolydian over it and wonders why it sounds a little bit weird, because that’s the standard thing to do over a chord. But it’s those extensions — that’s where the beauty is in the chord a lot of the time. And also, a lot of the defining notes that make it function properly.
The approach: start from the key, not the chord
Rather than assigning a scale to each individual chord in isolation, Joel’s method asks you to first understand the chord’s function within the key — whether it is acting as a tonic, subdominant, or dominant, and whether it is diatonic, a secondary dominant, a tritone substitution, or something else. From that functional context, the correct extensions follow logically. This is a significant shift in thinking for players who have learned to improvise chord-by-chord, but it is the approach that unlocks the sounds the bebop and post-bop masters were making — sounds that were never fully documented because the players had little incentive to explain their methods.
Why this knowledge was hard to find
As Joel points out, many of the pivotal players of the bebop era had every reason to keep their approaches private. Even the interviews that exist tend to be unhelpful: when asked what he practised, the reply “I just practice stuff I like” is essentially useless to a student trying to understand the approach. What Joel’s method provides is the systematic underlying logic that those players used intuitively, now made explicit and teachable in a form that guitarists — who rarely look to sax players for this kind of guidance — can apply directly.
Taking it further
Take one standard you know well and, rather than playing your usual scale choices, work out the function of each chord in the key and see what extensions that suggests. Do this slowly, away from the guitar, as a theory exercise first — then try applying it on the instrument. Notice where your previous choices were right for the wrong reasons, and where you might find new colours by choosing extensions that match the chord’s function.
Your homework
Choose a simple ii–V–I progression in a key you are comfortable with. Using Joel’s context-first approach, work out which extensions are diatonic to the key for each chord. Then practise playing through the changes using only extensions (9ths, 11ths, 13ths) as melodic targets. Record it and listen back to hear which choices create the most musical tension and resolution.