Mnemonics, up and down
Going up: My Bonnie Lies Over the Ocean leaps a major 6th on "My Bon-nie", and the NBC chime outlines the same interval on its first two notes. Going down: Nobody Knows the Trouble I've Seen opens with the fall, "No-bo-dy" dropping a 6th before it climbs back.
Here's the lesson attached. The pedagogue Gary Karpinski points out that the My Bonnie sixth runs from sol up to mi, but sixths also occur from do up to la, fa up to re, and elsewhere, and each placement carries its own pull within the key. One song can't cover them; in his words, there are several major sixths worth knowing. The mnemonic is a door in, not the room. That's also the design reason Aubel rounds keep changing the root: so the nine-semitone distance itself becomes the thing you recognize, wherever it sits.
Jazz keeps its own reference tunes. Mark Levine's interval table fixes the ascending major 6th to Monk's "Misterioso" and the descending one to Miles Davis's "All Blues."
On the staff
Wide but warm
Sixths sit with thirds in the imperfect consonances, the warm and blending intervals, and the kinship is literal: invert a major 6th and you get a minor 3rd, the same two pitch classes with the order flipped. That inversion is more than trivia. When a 6th goes by too fast to name, imagine the top note dropped an octave; if what remains is the easy, familiar minor 3rd, the original was a major 6th.
In melody the major 6th is a deliberate gesture. It clears the comfortable singing range of steps and small leaps, which is why composers save it for openings and arrivals that should feel like a stretch of the arms. Harmonized, sixths are the other default interval for two voices moving together; where parallel thirds sound close and conversational, parallel sixths sound roomier, the same warmth with more air in it.
What it gets confused with
The minor 6th is the near neighbor, one semitone smaller and noticeably darker, the same major-versus-minor discrimination as the thirds but spread wider, where it's harder. The inversion trick helps here too: a flipped major 6th leaves a minor 3rd, a flipped minor 6th leaves a major 3rd. The perfect 5th below it reads as hollow and finished where the 6th reaches; the minor 7th above it starts to rub and lean in a way no 6th does.
How Aubel handles it
The major 6th is the second interval to unlock beyond the starting set, right after the major 3rd, arriving once the whole pool ahead of it is mastered. Newly unlocked intervals get oversampled for their first fifteen or so attempts while the sound settles. The detail screen leads with My Bonnie Lies Over the Ocean, context caveat and all.