How it works. The top bar sets the key-center note (12-TET, A440 reference). The keyboard is a single polyphonic strip of the just-intonation pitches needed to spell barbershop vocabulary in the home key, spanning 1/2 (an octave below) to 5/2. Click a key to start/stop its pitch — any number can sound at once. Key sizes: big = the home major scale, medium = the default chromatic accidentals, small = temporary keys added for a selected chord root.

Chord roots. ⌘/Ctrl-click a key to make it the root of the chord you're spelling — it sounds, and (unless it's the key center) small temporary keys appear for every new pitch needed to spell barbershop chords on that root. While a root is active, every chord-tone key shows its name relative to the root (in the current label mode) in green above its home name. ⌘/Ctrl-click the root again to clear it; the temporary keys disappear.

Helmholtz–Ellis notation (text rendering). Note names are scale-degree numbers (1–7) relative to the reference, with a Pythagorean accidental (♯/♭) and comma marks: ↑ / ↓ = syntonic comma (5-limit, 81/80)  ·  ↟ / ↡ = septimal comma (7-limit, 64/63). Diatonic 3/5/7 (5/4, 5/3, 15/8) are kept full-size despite their commas; the just 5/3 is the canonical 6th.