common-tones

Common Tones Between Two Chords

When a user asks what notes two chords share, call ga_dsl_eval with closure name domain.commonTones. Don't compute the intersection mentally — the closure returns each shared note's interval role in BOTH chords (root / 3rd / 5th / 7th / 9th / 11th / 13th), and that role-mapping is exactly what the LLM gets wrong on extended/altered chords.

Why this skill uses ga_dsl_eval (not a keyhole tool)

This is the second canary for the DSL-eval pattern (PR #146 was the first, transpose). The domain.commonTones closure is already in GaClosureRegistry — exposed because its category is Domain. See docs/contracts/2026-05-06-ga-dsl-eval-contract.md (v0.1).

Calling the tool

Method: ga_dsl_eval

Args (flat key-value, all values as strings):

• closureName = "domain.commonTones"
• args.chord1 — first chord symbol, e.g. "Cmaj7", "F#m7".
• args.chord2 — second chord symbol.

Example invocation:

ga_dsl_eval(closureName: "domain.commonTones",
            args: { "chord1": "Cmaj7", "chord2": "Am7" })

Returns a DslEvalResult. Read Result for the formatted shared-notes string (or ResultJson for the same value as a JSON-quoted string). On failure, read Error.Code and Error.Message.

Mapping user phrasings

• "What notes do Cmaj7 and Am7 share?" → args = { "chord1": "Cmaj7", "chord2": "Am7" }
• "Common tones between G and D7" → args = { "chord1": "G", "chord2": "D7" }
• "Pivot tones from F to Bb7" → args = { "chord1": "F", "chord2": "Bb7" }
• "What's shared between Cmaj9 and Em7?" → args = { "chord1": "Cmaj9", "chord2": "Em7" }

Phrasing the answer

The closure returns a formatted string already. Surface it verbatim, then add a one-line interpretive hook tailored to the user's apparent intent (pivot, voice leading, modulation prep). Example:

Cmaj7 and Am7 share these notes:

• C → root of Cmaj7, minor third of Am7
• E → major third of Cmaj7, perfect fifth of Am7
• G → perfect fifth of Cmaj7, minor seventh of Am7

Three common tones — that's why Am7 is the canonical relative-vi substitute for C. Swap them anywhere in a progression with minimal harmonic disturbance.