Guitar Capo Chart mechanism

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

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