Harmonic Mirror
Play, and your instrument completes the chord you imply — adding the 1–2 notes you didn't play, tuned in pure just intonation to your root, so the harmony locks beat-lessly under your hands.
Or use your computer keyboard: a w s e d f t g y h u j k → C C# D D# E F F# G G# A A# B C.
Connect a MIDI piano and it drives the same pipeline. Idle ~2s and a demo arpeggiates I–vi–IV–V; your first note stops it.
Read the design notes
Your played notes are warm detuned-saw voices in equal temperament — the keys you actually pressed. The mirror infers your chord (maj / min / sus4 / bare-fifth / dom7 / cluster) and adds the missing 1–2 voices in just intonation relative to the root: 9/8 · 5/4 · 4/3 · 3/2 · 5/3 · 15/8. Because they are small-integer ratios over your root, they lock beat-lessly. When your root shifts, the mirror voices glide-retune to the new ratios. See the README for the full ratio table and the dated research citation.