Source: https://guthlabs.ai/docs/instruments/guitar-capo-chart/mechanism/

instrument mechanism / verified-clean

# Guitar Capo Chart mechanism

The inspectable method record for Guitar Capo Chart, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The core sounding-key calculation is standard under the stated semitone and pitch-class assumptions. Review is required because the specification does not define key spelling, chord-quality preservation, target-key suggestion behavior, valid fret inputs, guitar tuning assumptions, or the supported chord-shape table.

- The transposition formula is correct for equal-tempered semitone transposition, but openShapeKey is not defined as a canonical pitch-class mapping.

- The formula does not specify preservation of chord quality, extensions, slash-bass notes, or alternate voicings; only the root/key is transposed.

- Alternate shape-for-target-key suggestions have no formula or availability rule. The implementation must define whether barre and movable shapes are allowed and how unavailable or awkward shapes are ranked.

- Modulo 12 loses enharmonic spelling: pitch class 3 could be D-sharp or E-flat, for example. Key-signature and chord-spelling rules are required.

- The allowed capo-fret domain is unspecified. Negative, non-integer, or unrealistically high frets should be rejected.

## Units and conventions

- fret is an integer count of semitones; a capo at fret n raises every open-string pitch by n semitones.

- Pitch classes use C=0, C-sharp/D-flat=1, D=2, D-sharp/E-flat=3, E=4, F=5, F-sharp/G-flat=6, G=7, G-sharp/A-flat=8, A=9, A-sharp/B-flat=10, B=11.

- A capo at fret 0 means no transposition; a capo at fret 12 raises by one octave and preserves the pitch class.

- The chord's quality and extensions are preserved while its root is transposed.

- Target-key shape suggestions use shapePitchClass=(targetPitchClass-capoFret) mod 12 and require a separate chord-shape availability table.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "open_chord_shape": "C major",
    "open_shape_key": "C",
    "capo_fret": 2,
    "target_key": "E"
  },
  "expected_outputs": {
    "sounding_chord": "D major",
    "sounding_key": "D",
    "alternate_shape_for_target": "D major shape"
  },
  "working": "Use C=0 and E=4. Sounding pitch class=(0+2) mod 12=2 mod 12=2, which is D. Preserve major quality, so the sounding chord is D major. For an E-major target at capo 2: required shape pitch class=(4-2) mod 12=2 mod 12=2, which is D; therefore a D major shape produces E major."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The extreme-fret vector exposes unsafe JavaScript integer rounding and the absence of a practical fret-range validation check: mathematical input 9007199254740993 should produce A, but JavaScript Number rounding can make the calculation produce G# instead.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/guitar-capo-chart/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/guitar-capo-chart/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/guitar-capo-chart/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche6/tranche6-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`