Every key has a shadow twin. Negative harmony is a way of finding it. You take every note of a song and reflect it in a mirror placed at a fixed point in the key. Bright major chords come out dark and minor, and dark chords come out bright. The chords still pull towards "home" with the same force, though. The result follows the same musical story as the original, told upside-down. It often sounds eerie, bittersweet or cinematic.

The Swiss composer and theorist Ernst Levy developed the idea in A Theory of Harmony (1985). It builds on Hugo Riemann's 19th-century "harmonic dualism", which treats the minor triad as the major triad built downwards instead of upwards. Jacob Collier brought the idea to a wide audience in 2017, and it has been a favourite tool of composers and curious musicians ever since.

The mirror sits exactly halfway between the key's tonic (home note) and its dominant (the note a fifth above). In C major that is halfway between C and G, which lands between E♭ and E. Each note swaps places with its partner on the other side of the mirror:

In numbers, count notes in semitones (C = 0, C♯ = 1, … B = 11). Every note n becomes:

In C this is simply n′ = 7 − n, so C (0) ↔ G (7), E (4) ↔ E♭ (3), and so on.

Mirroring every note of a chord gives a new chord. Here are the basic chords of C major and their negative versions:

Some patterns jump out. The home chord turns minor (C → Cm), so a major scale becomes the minor scale on the same tonic. The dominant seventh G7, the chord that sets up the return home, becomes the dark Dm7♭5 (the same notes as Fm6). The classic "V → I" ending therefore turns into the "minor iv → i" sound heard at the end of countless film scores. The tension still resolves in the same way, just mirrored. In G7 → C, the note B rises a half-step to C and F falls a half-step to E. In the mirror image, A♭ falls a half-step to G and D rises a half-step to E♭.

1. Find the key. Every library song is labelled with its key. For a MIDI file the page estimates the key with the Krumhansl–Schmuckler algorithm. It adds up how long each of the 12 pitch classes sounds, then compares that profile against templates for all 24 major and minor keys, and the closest match wins. You can override the result if it guesses wrong.
2. Place the mirror. Put the axis halfway between the tonic and the dominant of that key.
3. Reflect every note with the formula above. In the library this applies to every note of every chord; in a MIDI file it applies to every note in every track: melody, bass, harmony, all of it. Drum tracks are left alone, because drums don't have pitches to reflect.
4. Rename the chords. The reflected notes are compared against a dictionary of chord shapes (major, minor, sevenths, sus, diminished and others) to give the new chord a name. In a strict mirror the old root ends up on top of the chord, so this page puts the new chord's own root in the bass to keep it sounding like a normal progression. That is how most musicians use negative harmony. A slash bass, such as D/F♯, gets reflected too.
5. Choose the octave. The mirror only says which note name to use, not which octave. In Keep register mode each new note lands in the octave closest to the original, so the bass stays low and the melody stays high. Pure mirror mode reflects the actual pitch, so rising melodies fall, falling ones rise, and the bass line ends up on top. It sounds strange, and it is fascinating.
6. Keep everything else. Rhythm, tempo, note lengths, loudness and the arrangement stay untouched. Only the pitches change, so the song stays recognisable.

Streaming services such as Spotify and Apple Music don't let other websites access a song's audio. Pulling every note out of a fully mixed recording is also still an unsolved problem. That's why the page works from chord charts (the library) and MIDI files (the full arrangement as notes). Free MIDI versions of most popular songs are easy to find online, and you can export one from notation software like MuseScore. Upload a MIDI file to hear a complete negative-harmony arrangement, melody included.