Structure Lab
Open Structure Lab →
What it’s for
Structure Lab shows you that the most complicated chord symbols are often two simple chords played at once. Put an A major triad over G7 and you have G13♭5; put a D minor triad over G7 and you have G9. Pick a chord, pick the triad on top, and Structure Lab tells you what the two make together, what it does harmonically, how to play it, and what to solo with over it.
Three things set it apart. Every combination has two grips worked out and fingered by hand, not generated, with the two chords shown in different colours so you can see each one inside the shape. Every combination is named three ways at once: as a stack (A over G7), as a chord symbol (G13♭5) and as harmonic function (IIMaj / I7). And the scale it gives you isn’t chosen from a list; it’s built from the notes of the two chords themselves, so it fits by definition.
Structure Lab is the interactive companion to the GMI book Upper Structures, Synonyms & Slash Voicings for Jazz Guitar.
Who it’s for
Players who already know their seventh chords and want the extended and altered sounds that sit above them, whether for comping, writing or soloing. If you can play a triad and a seventh chord, you can play everything in Structure Lab.
Free and Pro
Pro Structure Lab is a Pro app.
Free members and visitors who aren’t signed in see a short description of the app and the pathways strip above it, so you can read what each pathway covers before you upgrade.
Getting around
The page reads left to right. Choose the chord at the top, choose the triad that goes over it in the first column, then look at it in the second column and read about it in the third. Everything on the page rebuilds the moment you change any choice.
Above the app, the pathways strip offers four short guided routes: Stack a triad on a chord, Grips that are playable, The notes that fit and Into your own music.
The top bar
Save and Load, beside the Structure Lab title, keep the polychord you’re working on in your library and bring it back later. Save suggests a name from the polychord itself, for example “A / G dominant”. Structure Lab also keeps your place automatically: come back to the page and a ribbon offers to pick up where you left off, on any device you’re signed in on.
Below the title:
- Root — the root of the chord underneath, any of the twelve.
- Dominant, Major, Minor — the family of the chord underneath. Each button shows how many combinations that family has.
- Voice — Piano or Guitar for every Play button on the page.
- Foundation chord — the chord you’ve built and the notes it keeps under the triad on top.
The polychord
The first column is where you choose and see the combination.
Upper triad over G7 holds a tile for every triad that works over the foundation chord. Each tile shows the triad, its Roman numeral relationship to the chord underneath, and the chord the two make together. Click a tile to rebuild the page around it.
Under the tiles, the polychord is written as a stack, the triad over the foundation chord, with the same stack in Roman numerals beside it, so A over G7 sits next to IIMaj over I7. The numerals are the harmonic function, and they hold in every key.
Below that, the two chords are shown separately:
- Upper triad — its three notes, spelled as the triad itself (A, C♯, E). Under each note is the job it does in the finished chord: 9, ♭5, 13.
- Foundation — the notes of the chord underneath. When the triad brings an altered 5th (♭5 or ♯5), the foundation’s natural 5th is dropped, because the two can’t sit together.
Sounds as gives the chord symbol the two make, G13♭5, and the scale it belongs to.
Voicing and Pitch collection
The second column has two views, chosen with the tabs at the top: Voicing and Pitch collection. The page opens on Pitch collection.
Voicing
Every combination has two grips, each on its own card and headed with the string its root is on (Root on string 6, for example). These are fingered as they should be played, not as an algorithm would guess.
The diagram shows both chords at once. Notes from the foundation chord are blue, notes from the upper triad are purple, and a note that belongs to both is coloured half and half. The root has a red ring. Some grips leave the root out, as players often do with these chords; on those the root is shown as a hollow ring beside the neck so you can still see where it is.
Under the diagram, how it reads shows the grip in standard notation. A short note under some cards tells you something about that grip: “rootless”, “3rd omitted”, “top 5th optional”.
Each card has:
- ▶ Play — hear that grip.
- Sketchpad — add that grip to your Sketchpad collection.
- → Progression Lab — put that grip into a progression, with its exact shape kept.
Pitch collection
The pitch collection is every note of both chords put in order: the pseudo-scale. Because it’s built from the two chords, it fits the sound whether or not you can name it.
- Pseudo-scale — standard notation, with ▶ Play scale.
- TAB — the same notes on the neck.
- A row of the notes, each with its role in the chord and coloured by where it comes from: foundation, upper triad, or both.
Scales you can use then names the scale the collection sits in and its parent, for example “G Lydian dominant · D melodic minor · mode 4”. Choose a position with P1 to P7; the fretboard diagram shows it and ▶ Play position plays it. Where two scales both work over the chord, a pair of buttons above the positions switches between them. Whole tone and diminished scales repeat themselves across the neck, so for those Structure Lab explains the pattern instead of giving seven positions.
Theory + alternatives
The third column explains the combination.
- Why it works — what the triad adds to the chord underneath.
- Soloing — what to play over it.
- A note on the name — on the combinations where the naming needs explaining, such as why G13♭5 has a ♭5 and not a ♯11, or why Em over G7 is called G13 when there’s no 9 in either chord.
- On the staff — polychord — the two chords written out, the triad above the foundation, with the Roman numerals in the heading and ▶ Play polychord.
- Synonyms — other names the same notes go by, where there are any.
Sending the polychord
Under Send this polychord to, at the bottom of the page, Sketchpad sends both grips as two Sketchpad cards with the whole polychord stacked as the title, so both layers stay visible. To put a single grip into a progression, use → Progression Lab on the card you want.
Works with
- Chord Architect — a chord that Structure Lab covers gets a → Structure Lab button in Chord Architect. It opens Structure Lab on the matching polychord.
- Sketchpad — receives both grips from the send bar, or one grip from a voicing card.
- Progression Lab — receives a single grip from a voicing card, with its exact shape.
- My Library — your saved polychords, each with an Open button back into Structure Lab.
Related concepts
- Polychord
- Upper structure
- Upper structure triad
- Stacking chords
- Slash chord
- Extension
- Altered dominant
- Rootless voicing
- Synonym
- Lydian dominant
- Altered scale
- Whole tone scale
- Diminished scale
- Chord function
Try it
- Open Structure Lab. It starts on G7 with an A major triad on top: G13♭5.
- Press ▶ Play polychord, then switch to Voicing and play both grips. Find the A major triad inside each one: it’s the purple notes.
- Go back to Pitch collection, choose P1 and play the position. Then improvise over the G13♭5 grip using only those notes.
- Click the ♭VIMaj/I7 tile. Same G7 underneath, a completely different chord on top: G7♯5♯9. Play it and compare.
- Switch to Major, set the root to C and try D over Cmaj7: the Lydian sound.
