No mnemonic, on purpose
Every other interval page here opens with songs. This one can't: a unison is any note repeated, so every melody with a repeated note "contains" one. There's nothing to memorize. What's worth practicing instead is the discipline of hearing that nothing moved, because in a training round the temptation is to assume there must have been a leap and start hunting for one.
On the staff
The most consonant interval there is
Two voices on the same pitch fuse completely; in counterpoint the unison is so stable that the old rules confine it to beginnings and endings, since anywhere else the two parts just sound like one. It heads the perfect family (unison, 4th, 5th, octave), and its frequency ratio is 1:1, purity with nothing left to remove.
The interesting part is what happens just outside it. Two instruments almost on the same pitch produce beats: a slow pulsing that quickens as the pitches separate and dies away as they merge. Musicians tune by those beats. When an orchestra gathers on the oboe's A, every player is steering toward a unison by listening for the wobble to stop. Aubel's intonation game trains the same sensitivity from the other direction, asking whether a note sits sharp or flat of a reference.
How Aubel handles it
The unison is in the starting pool for every player, alongside the perfect 4th, 5th, and octave. It works as the control answer: when a round plays the same note twice, the right response is to trust your ear and take the zero. Players who internalize that early stop second-guessing the quiet rounds, and the habit pays off across the whole pool.