Sing 1-3-5-3-1 in two keys. Same shape, a fifth apart.

What’s happening. The tonic and dominant chords (I and V) are the two pillars of tonal harmony — between them they account for the harmonic motion in roughly 60% of all classical and popular music. Singing both arpeggios isolates the chord shape (1-3-5) from the chord root. Untrained singers can echo a tonic arpeggio from a cold pitch but struggle to transpose it: when the root moves to V, the shape often flattens because the internal map of “1-3-5” was tied to absolute pitches, not relative degrees.

Why I and V, why same shape both times. I and V differ by a P5 — the most consonant interval after the octave and the foundation of every cadence. Putting the same 1-3-5-3-1 shape on both keys makes the transposition lesson concrete: if your shape stays clean across the P5 shift, your ear hears relative degrees, not fixed pitches. ±50¢ with 4/5 per trial is generous on a per-arpeggio basis but tight in aggregate — you need 4/5 hits in both keys to claim mastery, which is the real harmony skill.

Why this is the capstone for ear-family. Everything beyond this is harmony — singing against a drone, resolving on the tonic after a cadence, harmonizing a melody. All of it assumes you can produce the I arpeggio and the V arpeggio cold, from any tonic. Lock both keys here and harmony-over-drone, a-cappella-resolve-tonic, and tonic-after-cadence become tractable. This is where singing transitions from pitch-matching to hearing chords.