Sketch a melody in C major. The harmonizer fits a chord under each note — one that contains the note, moves with a strong bass line, and lands on a cadence.
Tap a note, then tap a beat. Eight beats, one note each.
Each chord below contains the melody note above it. Roman numeral, chord name, and function (T = tonic, S = subdominant, D = dominant).
For every melody note there are three diatonic triads that contain it (as root, third or fifth). The harmonizer weighs them all at once: it favours the note sitting as a chord’s root or fifth, leans on the primary chords I, IV and V, rewards bass motion by a falling fifth, and pushes the final bar toward a cadence on I. “Another fit” nudges the weighting for a different but still valid harmonisation.