Mnemonics, up and down
Going up: Greensleeves opens with a minor 3rd, the leap into "A-las". Smoke on the Water's main riff makes the same move as its first change, and Brahms's Lullaby rocks up a minor 3rd as it settles in. Between a Renaissance tune, a hard-rock riff, and a lullaby, one of them is already in your head.
Going down: Hey Jude. The drop on the first two words is a falling minor 3rd, and McCartney lands it so plainly it might be the cleanest descending mnemonic in the whole set. As always, the fall is its own sound; the buttons above play the interval up, down, and stacked.
Jazz keeps its own reference tunes. Mark Levine's interval table fixes the ascending minor 3rd to Charlie Parker's "Confirmation" and the descending one to Dizzy Gillespie's "Groovin' High."
On the staff
The minor in minor
The minor 3rd is where minor keys get their name. Build a triad on any root: keep the perfect 5th, and the only choice left is which third goes in the middle. Three semitones and the chord is minor; four and it's major. Minor 7 and half-diminished chords sit on this interval, and stacking two minor 3rds produces the diminished triad.
Like all thirds it's an imperfect consonance: warm, blended, happy to keep moving. Don't let the "sad interval" cliché flatten it, though. The same three semitones open Greensleeves and Smoke on the Water, and neither is weeping. Context decides the mood; the interval just brings the color.
Where you'll keep meeting it
Every minor chord, by definition. The first two notes of a swung blues shuffle and the backbone of the pentatonic riffs guitarists learn first. Doorbells and "cuckoo" clocks tend to land near it. And it's everywhere in melody because it's the gap between so many neighboring chord tones: la up to do, mi up to sol, the top of every minor arpeggio.
What it gets confused with
The major 3rd is the one that matters, one semitone wider and the difference between minor and major. The imagined-chord test from that page works in both directions: heavy means minor, bright means major. The other early stumble is the major 2nd below it; a 2nd sounds like two adjacent steps of a scale, while the minor 3rd has a small but unmistakable hop in it, the first sound of a chord rather than a step in a line.
How Aubel handles it
Aubel teaches the minor 3rd late, the fifth interval to unlock beyond the starting set and well after the major 3rd. That's deliberate sequencing: by the time it appears, the major 3rd is familiar enough that the new interval registers as "like that, but darker," which is a much easier discrimination than learning both cold at once. The detail screen leads with the Smoke on the Water riff.