Mnemonics, up and down
Going up: the Star Wars main theme. After the opening pickup, the melody leaps a perfect 5th, and once you've noticed it you can't unhear it. Twinkle Twinkle Little Star makes the same jump at walking speed, from the first "twinkle" to the second.
Going down: the Minuet in G that everyone meets in piano lessons (Petzold, via the Bach notebook) opens with a drop of exactly a 5th, D down to G. Feelings falls the same distance on its first word. A falling 5th lands differently in the ear than a rising one, and the same seven semitones can sit in more than one place in a key, so the buttons above play it up, down, and stacked. A mnemonic gets the sound into your head. Naming it cold, from any root, is the skill the training builds.
Jazz keeps its own reference tunes. Mark Levine's interval table fixes the ascending perfect 5th to Milt Jackson's "Bag's Groove" and the descending one to Woody Shaw's "Katrina Ballerina."
On the staff
Neither major nor minor
The 5th sits in the perfect family with the unison, 4th, and octave: the intervals that come out identical whether a key is major or minor, the ones the earliest theorists held purest. The legend credits Pythagoras with finding the ratios on a vibrating string, and for the 5th the math holds: stop a string at two-thirds of its length and the pitch rises exactly a perfect 5th. A 3:2 ratio, the simplest in music after the octave's 2:1.
That purity has a practical edge. A bare 5th implies a chord but refuses to say which one; until a third arrives, major and minor are both on the table. C major is C-E-G and C minor is C-E♭-G, and the 5th is the same in both. It's also why the harmonic 5th sounds hollow next to a 3rd or a 6th: stable, open, slightly empty. That hollowness is the thing to memorize.
Where you'll keep meeting it
The violin, viola, cello, and mandolin all tune in perfect 5ths (the guitar goes the other way, mostly 4ths). The circle of fifths, the map every key signature hangs on, is this one interval applied twelve times. The bass motion that closes most tonal music, sol falling home to do at the cadence, spans a 5th. Bagpipe and hurdy-gurdy drones hold one under the melody for minutes at a time. And whenever a rock chorus hits a wall of distorted guitar that is somehow neither happy nor sad, that's bare 5ths doing what only they can.
What it gets confused with
The real mix-up is the perfect 4th, the 5th's inversion. Same family, same open and hollow color, and that similarity is what trips the ear: they're siblings, not just neighbors. The reliable difference is the sense of arrival. The 5th sounds settled, already home; the 4th leans, like a phrase that still owes you its ending. The major 6th, two semitones wider, reads as yearning and reaching where the 5th is content. And the octave is more open still, but it sounds like the same note twice rather than two different notes, so it rarely causes trouble.
A practical test: if you can sing a major triad up from the lower note and the interval's top note is sitting on the triad's roof, you've got a 5th. If the triad overshoots or undershoots, it's something else.
How Aubel handles it
The perfect 5th is in the starting pool for every player, alongside the unison, perfect 4th, and octave. The detail screen leads with the Star Wars theme and keeps Twinkle Twinkle as the backup for anyone who knows it better. Rounds play the interval from a different root each time, ascending, descending, and harmonic, so recognition stops depending on any single song.