# Music Theory

Notes, frequencies, intervals, scales, chords, keys, transposition, tempo math and Roman numeral analysis.

## How to pay

Machine-payable over x402. USDC on: Base (eip155:8453), Solana (solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp), Polygon (eip155:137), Arbitrum One (eip155:42161).
No accounts. No API keys. No signup. The paying wallet is the account.

## Machine-readable

- OpenAPI: https://music-theory.x402tookit.com/openapi.json
- x402 manifest: https://music-theory.x402tookit.com/.well-known/x402
- Catalog (free): https://music-theory.x402tookit.com/catalog
- Health (free): https://music-theory.x402tookit.com/health
- Uptime (free): https://music-theory.x402tookit.com/uptime

## Endpoints (10)

### GET /chord-name — $0.02
Name a chord from its notes (C E G Bb, A C E G): best name, inversion and alternative names; first note is the bass.
Params: notes: string
Example: https://music-theory.x402tookit.com/chord-name?notes=A+C+E+G

### GET /chord — $0.02
Spell a chord from its symbol (Cmaj7, F#m7b5, G7b9, Dsus4, C/E): notes, intervals, semitones and MIDI numbers.
Params: symbol: string
Example: https://music-theory.x402tookit.com/chord?symbol=Cmaj7

### GET /freq-note — $0.02
Frequency in Hz to the nearest note, MIDI number and offset in cents; adjustable A4 reference; tuner-style result.
Params: hz: number, a4?: number, accidentals?: string
Example: https://music-theory.x402tookit.com/freq-note?hz=442

### GET /interval — $0.02
Interval between two notes (quality, number, semitones) or the note a given interval above or below a note.
Params: from: string, to?: string, interval?: string, direction?: string
Example: https://music-theory.x402tookit.com/interval?from=C4&to=E4

### GET /key-signature — $0.02
Key signature (sharps or flats), circle-of-fifths neighbours, relative and parallel keys for any major or minor key.
Params: key: string
Example: https://music-theory.x402tookit.com/key-signature?key=Eb

### GET /note-freq — $0.02
Note name (C#4, Bb3) or MIDI number to MIDI, frequency in Hz and wavelength, with adjustable A4 reference pitch.
Params: note: string, a4?: number
Example: https://music-theory.x402tookit.com/note-freq?note=C4&a4=440

### GET /roman-numerals — $0.02
Roman numeral analysis of a chord progression in a key: numerals, inversions, borrowed chords, secondary dominants.
Params: key: string, progression: string
Example: https://music-theory.x402tookit.com/roman-numerals?key=C&progression=C+Am+F+G7

### GET /scale — $0.02
Spell a scale or mode on any root: major, minor, modes, harmonic and melodic minor, pentatonic, blues, whole tone.
Params: root: string, type?: string
Example: https://music-theory.x402tookit.com/scale?root=D&type=dorian

### GET /tempo — $0.02
BPM to milliseconds for note values, dotted and triplet delay times; or average tap intervals into a steady BPM.
Params: bpm?: number, taps?: string, beatsPerBar?: integer
Example: https://music-theory.x402tookit.com/tempo?bpm=120

### GET /transpose — $0.02
Transpose a chord progression by semitones or from one key to another, with spelling that fits the target key.
Params: progression: string, semitones?: integer, fromKey?: string, toKey?: string, prefer?: string
Example: https://music-theory.x402tookit.com/transpose?progression=C+Am+F+G7&fromKey=C&toKey=G

## Terms

Computed locally from standard Western music theory (12-tone equal temperament, scientific pitch notation with C4 as middle C and MIDI 60) and provided as is. Spelling conventions vary between sources and chord symbols are not standardised; results show the convention used. Educational and creative aid only, not a substitute for a teacher or a score.