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+# Chord Diagram Feature Design
+
+**Date:** 2026-04-09
+
+## Overview
+
+A chord diagram feature for the song detail page that shows users how to play each chord on piano or guitar. The core component is dumb — it receives only a chord name string and renders the diagram. All music theory and voicing logic lives in a separate library layer.
+
+## Architecture
+
+Three cleanly separated layers:
+
+```
+chord name string ("Cmaj7")
+        │
+        ▼
+  [theory layer]  — tonal parses name → root + note set {C, E, G, B}
+        │
+        ▼
+  [voicing layer] — maps note set → renderable positions
+      ├── Piano: note set → highlight keys on 1-octave keyboard
+      └── Guitar: chord quality + root → transpose moveable shape template
+        │
+        ▼
+  [render layer]  — dumb components, no music theory knowledge
+      ├── <PianoKeys notes={["C","E","G","B"]} />
+      └── <GuitarFretboard frets={[x,3,2,0,1,0]} baseFret={1} />
+```
+
+## Files
+
+```
+app/app/
+  lib/
+    chord-voicing.ts        # theory layer: tonal → notes + guitar voicing
+    guitar-voicings.ts      # data: ~25 quality templates
+  components/
+    chord-diagram/
+      piano-keys.tsx        # dumb renderer: string[] of note names → keyboard
+      guitar-fretboard.tsx  # dumb renderer: frets[] + baseFret → fretboard grid
+      chord-diagram.tsx     # entry point: chord+instrument → voicing → renderer
+      chord-grid.tsx        # wrapped grid of ChordDiagram cards for all song chords
+```
+
+## Component API
+
+### `<ChordDiagram>`
+
+```tsx
+<ChordDiagram chord="Cmaj7" instrument="piano" />
+<ChordDiagram chord="Am" instrument="guitar" />
+```
+
+Renders nothing (graceful empty) if the chord cannot be parsed or has no voicing.
+
+### `<ChordGrid>`
+
+```tsx
+<ChordGrid chords={["Em7", "G", "Dsus4", "Am7"]} />
+```
+
+Owns the `instrument` state (`"piano" | "guitar"`), persisted to `localStorage` as `chordDiagramInstrument`. Renders a global piano/guitar toggle and a `flex-wrap` grid of `<ChordDiagram>` cards.
+
+## Diagram Styles
+
+- **Piano:** dot notation — white keys with filled circles on pressed keys, black keys overlaid. 1 fixed octave shown (C to B); notes are matched by name regardless of octave.
+- **Guitar:** standard vertical fretboard — nut at top, 4 frets shown, dots on finger positions, O/X above strings for open/muted. Barre indicator where applicable.
+
+## Theory Layer (`chord-voicing.ts`)
+
+Uses `@tonaljs/tonal` (already in npm, tree-shakeable):
+
+```ts
+export function getPianoNotes(chord: string): string[]
+// "Cmaj7" → ["C", "E", "G", "B"]
+// Returns [] if unparseable
+
+export function getGuitarVoicing(chord: string): GuitarVoicing | null
+// "Am" → { frets: [0,0,2,2,1,0], baseFret: 1, barre: null }
+// Returns null if quality not in voicing map
+```
+
+## Guitar Voicing Data (`guitar-voicings.ts`)
+
+~25 moveable barre-chord templates keyed by `tonal` chord type name. Each template is a barre shape (no open strings) so it can be transposed by shifting `baseFret`. Two shape families are used: E-shapes (root on 6th string) and A-shapes (root on 5th string). `baseFret` is computed as the semitone distance from the template shape's root string pitch (E or A) to the target chord root.
+
+```ts
+interface GuitarVoicingTemplate {
+  frets: (number | null)[]  // 6 strings; null = muted; fret numbers relative to baseFret
+  baseFret: number          // 1 in template; shifted when transposing to target root
+  barre: number | null      // fret (relative to baseFret) to draw barre, or null
+  rootString: 'E' | 'A'    // which string carries the root (determines transposition offset)
+}
+```
+
+Quality names match `tonal`'s `Chord.get(name).type` output (e.g. `"major"`, `"minor"`, `"major seventh"`, `"dominant seventh"`, `"minor seventh"`, `"diminished"`, `"augmented"`, `"suspended fourth"`, `"suspended second"`, `"half-diminished"`, `"dominant seventh flat five"`, etc.). ~25 entries total.
+
+If `tonal` returns a quality name not in the map, `getGuitarVoicing` returns `null` and the diagram renders a "no guitar voicing" placeholder.
+
+## Layout & Integration
+
+### Breakpoint
+
+`lg` (Tailwind) divides mobile from desktop layout.
+
+### Mobile
+
+- Below `lg`: lyrics and diagrams in a single column.
+- **Inline popup:** tapping a chord name in `chord-chart.tsx` sets `activeChord` state in `songs.$id.tsx`. An inline `<ChordDiagram>` panel appears immediately below the tapped line. It closes when the scroll container fires a `scroll` event.
+- **Bottom grid:** `<ChordGrid>` rendered after `<ChordChart>` in the scroll column. Not sticky — scrolls with content.
+
+### Desktop
+
+- At `lg` and above: `songs.$id.tsx` switches to a two-column layout.
+- Left column: `<ChordChart>` (existing).
+- Right column: `<ChordGrid>` showing all unique chords in the song, wrapped. No inline popup on desktop (side column is always visible).
+
+### Chord list source
+
+`songs.$id.tsx` derives `uniqueChords: string[]` from `displayed.sections` — all unique chord names in order of first appearance, deduplicated. This list is passed to `<ChordGrid>` and also used to determine which chord names in `<ChordChart>` are tappable.
+
+### Instrument toggle
+
+Global piano/guitar toggle lives in `<ChordGrid>`. State persisted to `localStorage` as `chordDiagramInstrument`. Switching updates all visible diagrams at once.
+
+## Error / Unknown Chord Handling
+
+- `getPianoNotes` returns `[]` → `<PianoKeys>` renders with no dots highlighted and a subtle "?" label.
+- `getGuitarVoicing` returns `null` → `<GuitarFretboard>` renders an empty fretboard with a "no voicing" label.
+- Unparseable chord name (garbage string) → same fallback as above.
+
+## Dependency
+
+Add `tonal` to `app/package.json`. It is tree-shakeable; only chord parsing and note utilities will be bundled.