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Diatonic Key Builder Guide

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Diatonic Key Builder

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Diatonic Key Builder showing G major in Drop 2 on strings 2 to 5: seven chord cards climbing the neck, each with its Roman numeral, name, notes and diagram.
G major, Drop 2, strings 5-4-3-2

What it’s for

Diatonic Key Builder shows you every chord in a key, in the inversion you choose. Send it any three- or four-note chord from Chord Lab, Chord Architect or Progression Lab, and it builds the whole key around that shape, up the neck or across every string set. It’s the quickest way to see a key as shapes under your fingers rather than as a list of names.

Who it’s for

Anyone learning how keys and chords fit together: players who want to know which chords “go” in a key, songwriters looking for the next chord, students of modes, and anyone building a chord vocabulary up and across the neck.

Free and Pro

The Diatonic Key Builder page for a free member: the pathway strip above a panel reading "GMI Diatonic Key Builder — A Pro feature" with an Upgrade to Pro button.
What free members and visitors see

Pro Diatonic Key Builder is a Pro app. Free members and visitors see what it does, can follow the pathway strip, and can upgrade from the page.

Compare Free and Pro

Getting around

From the top of the page down:

  • The GMI app dock links to every other app.
  • The pathway strip offers four guided walkthroughs.
  • The selector bar: Key, Mode / Scale, Voicing and String Set.
  • The mode note underneath says what the scale or mode is, and its other names.
  • The action bar: Neck View and String Sets on the left; ↗ Harmonic Universe, → Send to Progression Lab, ▶ Play Sequence, → Practice Studio, Save and Load on the right.
  • The chord cards, one for each chord in the key.

A quick way in: leave the key on G, choose Major scale, and press ▶ Play Sequence.

Choosing a key

Key sets the starting note: any of the twelve. Everything updates as soon as you change it. The key you’re in shows beside the app’s title. The ? beside Key explains the selectors.

Related: diatonic, key signature, relative keys

Modes and scales

Mode / Scale chooses which scale the chords are built from. The list is grouped:

  • Parent scales: the Major scale, Harmonic minor and Melodic minor
  • Modes of the major scale: Dorian, Phrygian, Lydian, Mixolydian, Natural minor and Locrian
  • Modes of harmonic minor: Locrian ♮6, Ionian ♯5, Dorian ♯4, Phrygian dominant, Lydian ♯2 and Altered diminished
  • Modes of melodic minor: Dorian ♭2, Lydian augmented, Lydian dominant, Mixolydian ♭6, Locrian ♮2 and Altered
  • Symmetrical diminished: Half-whole diminished and Whole-half diminished

Below the selectors, the mode note tells you where the mode comes from and what else it’s called. For example: “Dorian — Mode II of the major scale.” or “Phrygian dominant — Mode V of harmonic minor. Also called Phrygian ♮3, Spanish, Freygish or Mixolydian ♭2♭6.”

Related: major scale, harmonic minor, melodic minor, modes, Dorian, Mixolydian, Lydian dominant, Phrygian dominant, altered scale, diminished scale

Voicing and string set

Voicing chooses how every chord is played: Drop 2, Drop 3, Close triads, Shell Voicings or Drop 2+4. With Close triads the chords are three-note chords (C, Dm, B° and so on); with the others they’re seventh chords.

String Set chooses which strings the chords sit on. The choices follow the voicing:

  • Drop 2 and Shell Voicings: strings 4-3-2-1, 5-4-3-2 or 6-5-4-3 (shell voicings: 4-2-1, 5-3-2 or 6-4-3)
  • Drop 3 and Drop 2+4: strings 5-3-2-1 or 6-4-3-2 (drop 2+4: 5-4-2-1 or 6-5-3-2)
  • Close triads: strings 3-2-1, 4-3-2, 5-4-3 or 6-5-4

Related: drop 2, drop 3, drop 2 and 4, shell voicing, triads, string set

The chord cards

Three chord cards from G major, Bm7, Cmaj7 and D7, each with its coloured Roman numeral, chord name, notes, inversion, diagram, function line and buttons.
The chord cards

There’s a card for each chord in the key, in order, climbing the neck. Each card shows:

  • The Roman numeral, coloured by degree. Click it to make that chord the new home: the key changes so this chord becomes I.
  • The chord name, and its notes underneath.
  • The inversion of the shape shown.
  • The diagram, with notation and TAB.
  • What the chord does in the key, in Neck View: for example “Tonic — home, rest and resolution” or “Dominant — strong pull back to I”.

Then the buttons:

  • Chord Lab opens this exact voicing in Chord Lab. It appears on major 7, minor 7 and dominant 7 chords, which are the ones Chord Lab shows.
  • + Prog adds this one chord to Progression Lab. You stay where you are.
  • Sketch adds the chord to your Sketchpad. If it’s already there, you’re told so rather than getting it twice.
  • ▶ plays the chord.

Between the cards, a small dot lights up when two neighbouring chords share notes; hover over it to see how many.

After card VII, a final card brings you back to the tonic, an octave up, to close the loop. If there’s no room for it on the neck, card I is shown again in its place, labelled I with “Returns to the tonic — closes the diatonic loop”.

Related: chord function, inversion

Neck View and String Sets

String Sets view of G major in Drop 2: the row for strings 1 to 4, eight cards from Gmaj7 back to Gmaj7, each with its inversion and diagram.
String Sets view

Neck View (the default) shows the key on the string set you’ve chosen, with the chords climbing the neck.

String Sets shows the key on every string set for that voicing at once, one row per set, so you can compare them. In this view the String Set selector is hidden (every set is on screen) and Row Layout appears instead:

  • In position: each row keeps the chords close together in one area of the neck.
  • One inversion, up the neck: each row keeps the same inversion and moves it up the neck.

Each row has its own ▶ to play that row. A string set that can’t hold the key in that voicing is left out. Click anywhere in a row to make it the one that → Send to Progression Lab and → Practice Studio send.

Related: positional playing, position shifting

Playing the key

▶ Play Sequence plays the chords in order, lighting each card as it sounds. Press it again to stop. Changing the key, mode or string set also stops it.

Chords arriving from other apps

The Chord in context row after sending Dmin9 from Chord Architect: buttons for Dorian, Natural Minor, Phrygian, Major (II) and Major (III), the line "Dmin9 is degree I of D Dorian", and the D Dorian cards below.
A chord sent from Chord Architect, shown in context

You can start the Key Builder from a chord in another app: ⬡ Build Key in Chord Lab and Progression Lab, or → Build Key in Chord Architect. It’s offered for shapes on four strings or fewer. The Key Builder opens on a key that contains your chord and scrolls down to the cards. Your chord’s card uses your exact shape and name; the other cards are voiced from GMI’s chord library, each one the nearest playable shape moving up the neck from the card before.

When the chord comes from Chord Architect, a row headed Chord in context: appears above the cards. Each button is a key or mode the chord belongs to; click one to rebuild the key around it. The line underneath says where your chord sits, for example “Dmin9 is degree I of D Dorian”.

Change the key, mode, voicing or string set yourself and the Key Builder goes back to building the key normally.

Saving

Save keeps your setup (key, mode, voicing, string set and view) in your library under a name you choose, and Load opens a saved one. The dot beside them shows whether your current setup is saved. Your saves also appear in My Library.

If you were last working on another device, a note above the cards offers to take you back there.

Sending on

  • → Send to Progression Lab sends the whole key, in order, and opens Progression Lab. The chords arrive with the names shown on the cards.
  • → Practice Studio sends the key to Practice Studio and opens it.
  • ↗ Harmonic Universe opens this key in Harmonic Universe, to see how it connects to other keys. For a mode, it opens the parent key: D Dorian opens C major. It isn’t shown for the diminished scales.

Pathways

The four Diatonic Key Builder pathways: Building a key, Modes and their parents, Voicing the key, and Building app connections.
The pathway strip

Four guided walkthroughs:

  1. Building a key: the selectors, the cards and playback
  2. Modes and their parents: the mode list and the mode note
  3. Voicing the key: voicings, string sets and the two views
  4. Building app connections: sending to and from the other apps

Works with

Diatonic Key Builder receives from:

  • Chord Lab: ⬡ Build Key on a chord card, for shapes on four strings or fewer
  • Chord Architect: → Build Key in the chord panel, for voicings on four strings or fewer
  • Progression Lab: ⬡ Build Key in a chord’s enlarged view
  • Harmonic Universe: Send to DKB in the Selection tray, which opens the Key Builder on the chosen chord’s key

Diatonic Key Builder sends to:

Diatonic · Chord function · Major scale · Modes · Relative keys · Drop 2 · String set · Inversion

Try it

Choose C, Major scale, Drop 2, strings 2–5, and press ▶ Play Sequence. Then change the mode to Dorian, keep the key on C, and play it again: same root, a different set of chords. Finally, switch to String Sets to see the key on every string set at once.

Open Diatonic Key Builder →

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