Sixths
A bigger interval than a third — 8 or 9 semitones instead of 3 or 4 — and a hand shape most players find needs a little more time to settle in. The interval itself — sixths, everywhere in real harmony as inverted thirds — is worth the investment.
The shape: same low/high logic, a wider sound
How to play it
Let the hand widen from the base knuckles, not by straining the fingertips forward. If the hand feels cramped, the frame is wrong before the pitch even matters.
Same tuning method, a bigger target to miss
The wiggle-to-lock check from thirds applies here too. A bigger interval gives the ear less
to compare the two notes against directly — it's easier to be a little off and not notice
until you actually listen for the beating.
Try it
Try
Play the minor sixth, then the major sixth, keeping the 1st finger anchored on A the whole time and only adjusting the 2nd finger's low/high position. Feel how little that finger has to move for such a clear change in colour.
The usual mistake
Tensing the hand while reaching for the low/high choice on the upper finger, instead of letting it stay loose. A gripped hand can't make the small adjustment that tells major from minor — tension there costs you precision, not just comfort.