0 3 7 — music theory lab for tracker people

A one-file browser app that teaches music theory in semitones — no staff notation, nothing you'd have to translate before typing it into a tracker. Built for chiptune / tracker musicians, and named after the question that started it:

Is the chip-arp formula 0 3 7 the right chord on every note of the scale?

No. It's the minor triad. Stack thirds through the scale and the formula changes with the degree — that's the whole idea in one table (C major):

Digits that were bent to stay in scale show amber in the app; anything that leaves the scale shows red. Every chord, every scale, every key.

open 037-lab.html

No build, no server, no dependencies — works from file:// and offline (the only network request is the JetBrains Mono web font, and it degrades gracefully). Sound starts on your first click; everything is synthesised in the Web Audio API: pulse, triangle and saw waves plus a shift-register noise for the drums. Chrome, Firefox and Safari work; Web MIDI needs Chrome, Edge or Opera.

The header holds the root, scale, waveform, arp rate and volume for everything below, a MIDI button where the browser supports it, and Reset. The address bar always holds a link that restores the key, every setting, the seed and the exact pattern (Copy link puts it on the clipboard), and the last state comes back when you reopen the page. Reset forgets all of that and reloads with the defaults (A minor, a fresh pattern); the Ear tab's scores are kept.

• 24 scales in four families (the modes; harmonic, melodic, Hungarian, double harmonic and other minors; pentatonic, blues, hirajoshi, in-sen, yo, Ryukyu; whole tone and both diminished), each with a one-line mood. A row of chips lists the keys that share the same notes (A minor = C major = D Dorian …) and the parallel key; click one to switch.
• A keyboard that lights the scale. Play it with the mouse, with the computer keyboard the way trackers do (Z–/ one octave from C-3, Q–P the octave above, [ ] move it), or from a MIDI keyboard after pressing MIDI.
• Every chord shape (triads, sus, power, 7ths, 6ths, add9) on every scale note, as the digits you type into the arp. Snap to scale shows the raw formula vs the diatonic one. Every card lists its inversions; pick a Top note and the inversion that puts it on top lights up in every card, since a fast arp is heard by its highest note.
• Common progressions as fast arps, one bar each, plus Your progression from the Melody Lab.
• Finder — the reverse question. Type a formula you copied from someone's song (0 5 9), note names (C E G) or tracker notes (C-4 E-4 G-4), or tick Pick on keyboard and play the notes: the left pane names the chord, its inversion and root-position formula, lists close chords when nothing matches, and shows which keys hold it and as which degree. The right pane is the scale finder: type all the notes of a song and it ranks every scale that holds them by fewest unused notes, names the unused ones, and Use sets root and scale.

A melody generator that thinks like a tracker: 16 rows per bar, hex row numbers, C-4 notes, | for a note still ringing, ··· for a rest. Pick length (1–8 bars), tempo, density, range, rests and leaps; it writes chord tones on the strong rows, walks stepwise between them and shapes phrases (A B A B′; longer forms repeat and end on a cadence). Every Generate has a seed: same seed, same settings, same melody. Undo / Redo cover everything. Space plays.

• Columns: Melody, Arp (the chord channel: one note per bar plus Jxy, the tracker arpeggio command with the chord's offsets; Voicing picks the inversion, auto keeps the top note close from bar to bar), Bass (a triangle octave bass), Drums (a chip noise kit — kick, snare, hat, open hat from a 15-bit shift register; six Drum styles, a fill in the last bar) and Chord (the chord per bar; click it for alternatives).

• Progression — type your own, one chord per bar, up to eight; changing it re-chords the pattern under the existing melody:

  you type	you get
  1 5 6 4	scale degrees, snapped: whatever triad the scale gives on that degree
  i VI III VII · I V vi IV	roman numerals mean what they say: upper = major, lower = minor
  57 · V7 · 2m7 · 1maj7 · 4sus4 · 7dim	a suffix forces the shape: m maj dim aug sus2 sus4 5 7 m7 maj7 m7b5 dim7 6 m6 add9
  b6 b7 · #4dim · bVI	b/# shift the root a semitone — borrowed chords; digits that leave the scale show red
  1 - 5 -	- holds the previous chord for another bar

• Slides — Some or Lots places tone portamento where it belongs (up into the peak, down into the cadence, across leaps). The amber 3xx column is the value to type, timed like SID-Wizard's calculated slides; Hear slides A/Bs the same melody without them.

• Edit like a tracker — click a cell in the Melody, Bass or Drums column, then:

  key	does
  ↑ ↓	move the note through the scale (bass: the bass range; drums: kick → snare → hat → open hat)
  Shift ↑↓ · Alt ↑↓	an octave · a semitone (outside the scale turns red)
  R · Del · Enter	reroll · mute to a rest (double-click too) · start a note, or split a ringing one
  + − · S	lengthen / shorten · toggle a slide into the note
  ← → Tab Esc · Ctrl+Z Ctrl+Shift+Z	move around, deselect · undo, redo

  Hand-edited bass and drums survive a progression change and travel in the share link.

• Export — every column header has IT and SWM buttons that copy the column to the clipboard in OpenMPT's ModPlug Tracker IT text (11-character cells, === note-off, Gxx slides, Jxy arps) or DUET's DUET SW text (9-character cells, 03xx slides, 07xx chord select with the chord table listed for you). Drums export as a fixed note with instruments 01–04. Copy as text gives a plain readable dump.

• Paste from your tracker — the other direction. Copy a channel in OpenMPT (IT or XM), Schism or DUET and press Paste from tracker (or the ⇩ button in the Bass or Drums header). Notes, note-offs, held notes and tone portamento come back as cells and slides; other effects are counted and ignored. If the browser will not hand over the clipboard, a text box appears.

• Suggest chords — harmonizes whatever is in the Melody column: scores every chord of the scale per bar by how well it fits the notes (strong rows and long notes weigh more, a hand-made bass counts as the root), then picks the sequence that moves well and lands on the tonic. The result becomes the progression text; the Chord column offers alternatives per bar. Pasted melodies get this automatically.

Ear training in semitones. Intervals (easy 3 4 5 7 12, medium, or all twelve; up, down, together or mixed), Chords (triads; plus sus and power; plus 7ths; as a chord or an arp), Scales (the common six or all 24) and Degrees: the tonic chord of the current key, then one note — name the scale step, the skill that writes melodies. Answer by click or keyboard (1–9 0 - = are 1–12 semitones or degrees, or the n-th chip), R replays, Enter asks the next one. A wrong answer plays the right one and then yours. Scores per mode stay in the browser.

The chip tricks that make three channels sound like a band, most with a Hear it button: chord arps and where the arp rate comes from (ticks, tempo, multispeed), inversions for the top note, fake echo, delayed vibrato, slides, hard restart, octave bass, noise drums, detune, PWM, sidechain pump, one channel doing two jobs, secondary dominants, borrowed chords, the semitone-up key change, instrument design, arrangement in 4s and 8s.

Semitones as the only unit, scales as a palette, the circle of fifths as counting by 7, why chords aren't always 0 3 7, inversions, progressions, tension and release, writing melodies, harmonizing a melody, rhythm & groove, and a cheat sheet with every formula, the progression syntax, the exported tracker commands and every keyboard shortcut.

Everything is a semitone offset. Notes are MIDI numbers under the hood and tracker names on screen (C-4, A#3 — sharps only; the IT export calls the same note C-5, as OpenMPT does). Scales and chords are arrays like [0, 3, 7]. Roman numerals only appear for 7-note scales. Colours: cyan = action, amber = adjusted to fit the scale, red = outside the scale, faint = holds and rests.

It really is one file: <style>, the markup, then a <script> split by /* ===== name ===== */ banners. The interesting bits are data tables — SCALES (offsets, family, mood), CHORDS (the raw formula plus which scale steps it stacks, which is what makes snapping work for any scale length), PROGS, DRUMS and DRUM_PATS, COLS (the pattern columns), EAR_MODES, and TECH / THEORY (the cards; a d: key wires a Hear it button to DEMOS). fitChord() is the heart of it; voice(), noise() and kick() are the synth primitives, and all note timing runs on AudioContext time — never setTimeout. CLAUDE.md maps every subsystem. Edit, reload, done.