My initial plan when building the Music Theory API v1 was to include figured bass notation as a field in the chord endpoints.

The idea was straightforward: return the standard numeric figures alongside the chord notes so that the response would be useful for anyone working with Baroque-era notation or voice leading analysis.

The initial json implementation looked like this:

{

"root": "C",

"quality": "minor",

"name": "C minor",

"inversion": "first inversion",

"bass": "Eb",

"notes": ["Eb", "G", "C"],

"basso_continuo": "6/3"

}

For root position, first inversion, and second inversion triads, the figures map cleanly to inversion number: root position is 5/3, first inversion is 6/3, second inversion is 6/4. For seventh chords the same logic applies: root position is 7/5/3, first inversion is 6/5/3, second inversion is 6/4/3, third inversion is 6/4/2. This works until you try a diminished triad in root position.

A diminished triad has a diminished fifth above the bass, not a perfect fifth.…