How different would songs sound if they were written in the opposite key?

If songs were rewritten using negative harmony as their "opposite key," they would sound significantly different but still retain some structural familiarity. Negative harmony, a concept popularized by Ernst Levy and later explored by musicians like Jacob Collier, involves inverting the harmonic relationships around a central axis (often the tonic).

How It Would Change the Sound:

• Chord Inversions & Reinterpretation – Major chords would transform into minor chords and vice versa, but not in a simple direct swap. Instead, each note moves symmetrically around the tonal center. Example: In C major, a G (V) chord (G-B-D) might turn into a D♭ (♭II) chord (D♭-F♭-A♭).
• Emotional Shift – Songs that originally sounded bright and uplifting could take on a darker, more mysterious, or melancholic tone. Conversely, sad songs might gain an unexpected brightness.
• Melodic Transformation – The melody itself would shift, as each note in the scale is mapped to its opposite around the tonal center. This can create an eerie familiarity, as if the song exists in a "mirror dimension."
• Unusual Resolutions – Cadences and harmonic resolutions would behave differently, leading to progressions that sound less conventional but still retain internal logic.

Examples:

• If you applied negative harmony to "Let It Be" (C major), its major sound would morph into something more solemn and reflective, perhaps in G minor or D♭ major with unexpected harmonic twists.
• A dark song like "Hurt" by Nine Inch Nails in A minor might sound unexpectedly uplifting in its negative harmony counterpart.

Tools to find Negative Harmony:

You can use the Negative Harmony tool available in the Music Theory Companion mobile app for figuring out Negative Harmony of every chord and notes for any Key.

The screenshot above taken from the Music Theory Companion app that shows Negative Harmony notes and chords for the C Major scale.

Conclusion:

Songs rewritten in negative harmony wouldn't just be "major becomes minor" but rather a full harmonic inversion that feels both alien and oddly recognizable. The emotional core of the song would shift, often in an uncanny, dreamlike way.