Two flats, and why one of them is fatal
Locrian lowers two degrees against the natural minor it is closest to: the 2nd and the 5th. The flat 2nd it shares with Phrygian: a half step above the tonic, dark and tense. The flat 5th is the one that sets Locrian apart from every other mode. A perfect 5th above the tonic is what every other scale uses to build a stable home chord; flatten it and the tonic triad becomes diminished, a chord that by its nature cannot rest. So Locrian is permanently unresolved. It does not arrive; it hovers, off balance.
On the piano it is the white keys from B to B: the 7th mode of the major scale, C major's notes centered on B. The step pattern, H W W H W W W, packs both half steps into the lower half, leaving the top oddly flat and unsupported.
On the staff
The mode that barely shows up
It is worth being honest about Locrian: it is hardly ever used as a key. Theory texts name it and move on; C&M lists it as occasionally used and leaves it largely undescribed. The reason is the diminished tonic: music that wants a place to land cannot land here. Where Locrian does appear is where that instability is the point: a handful of metal and experimental pieces use it precisely because it refuses to settle, building tension that never pays off into a home chord.
There is one place it earns steady work. Kostka and Payne note that jazz players use Locrian as a basic structure for improvisation, and Mark Levine names it the bebop chord-scale for the half-diminished (m7♭5) chord. So while Locrian almost never anchors a key, it is alive every time a soloist plays over a half-diminished chord.
For ear training, then, the goal is not to spot it in your favorite songs, which you mostly will not, but to recognize the combination that makes it: a dark flat 2nd like Phrygian, plus a tonic that never finds its footing. If a scale sounds like Phrygian but somehow even less grounded, you are hearing Locrian.
How Aubel handles it
Locrian unlocks last, the ninth and final scale, the end of the mode ladder. It belongs there: it is the hardest to pin down precisely because it never resolves, and it is easiest to confuse with Phrygian, which it sits right next to in the progression. The distinguishing cue is the flat 5th and the ungrounded feel it produces. Rounds play the full scale up and down from a random root, so what you are training is that signature instability rather than any one key.