E Dorian and A Dorian. A fourth apart, and they differ by exactly one note — C♯ becomes C. That gap is the whole reason to pick this pair: if two areas sit far apart you just move your hand and play a different lick. Here there's nowhere to hide.
Both scales sit in the same box, frets 4 to 9. Toggle between them and watch what happens — almost nothing moves. Three notes slide back a single fret and that is the entire physical difference between the two key centres.
The scale shape is otherwise identical, which is the point. Your fingers already know both areas — what you're actually practising is the ear and the timing of a single note change, not a new fingering. That's a much harder thing to get right and a much more useful thing to own.
If you want one anchor to hold on to and nothing else: C♯ on the G string, fret 6. C natural on the G string, fret 5. One finger, one fret, the whole switch.
Two bars per chord, eight bars round. The chords have to carry the colour tone, or both areas collapse into E minor pentatonic and you learn nothing from playing over them.
The A chord in bar 3 carries the C♯ that forces E Dorian rather than E Aeolian. The Am9 in bar 5 carries the C natural that forces the new centre. Those two chords are doing all the work — swap either for something vaguer and the exercise stops functioning.
Play nothing but C♯ in the E bars and C natural in the A bars. Land on them, hold them, let them ring into the chord. It will feel absurdly restrictive and it is the most valuable pass of the three — you're teaching your ear what the switch sounds like before your hands get a vote.
Add the minor pentatonic around the colour tone, but every phrase must end on it. Start anywhere, finish on C♯ in the first half and C in the second. Phrase lengths of your choosing — the only rule is where you land.
Play whatever you want. The single measure of success is that you never play C♯ over the Am9 and never play C natural over the A. Nothing else about the solo matters yet.
You will carry the C♯ into the A section for a beat or two. It sounds like a mistake because it is one — C♯ over Am9 is a ♭9 against nothing, no tension that resolves, just wrong. If you can make the change land cleanly on the downbeat of bar 5, the exercise has done its job and you can move on.
Eight bars at 80bpm is about 24 seconds — comfortably inside the TimeLine's 30-second looper. Record the four chords once, let it run, and solo over the top. The loop returns through the Mesa while you play live through the Badger, so the rhythm part sits in a different place in the room from the lead.
That separation matters more than it sounds. You can hear the chord change from across the room, which is exactly what you need when the whole exercise turns on catching the downbeat of bar 5.
Start at 80bpm and stay there for a week. The failure mode is speeding up so the switch arrives before you've had time to hear it coming — at which point you're practising reaction, not anticipation. When you can nail bar 5 ten times running, drop to 60 rather than going faster. Slower is harder here.
Keep E Dorian, swap the second area for E Mixolydian — same root, G becomes G♯. Now the root doesn't move and there's no chord change to warn you, so the switch is pure ear. That's the Henderson version and it's considerably less forgiving.
The other direction is to keep the two Dorians and shorten the bars: four bars each, then two, then one. A switch every bar at 80bpm is a genuinely difficult thing to do cleanly and it's the point at which this stops being an exercise and starts being playing.