Music theory · 6 min read

How chords are built: triads and sevenths

Chords are stacked thirds. Here is how the four triad types and the main seventh chords are constructed, named, and found under your own hands.

2026-05-28

Stacking thirds

Take any scale and choose a note. Skip the next letter and take the one after, then skip again and take the next. From C in the C major scale you get C, E, G. That is a triad, so called because it has three notes, and it is the basic unit of Western harmony.

The bottom note is the root and gives the chord its letter name. The middle note is the third, because it sits a third above the root. The top is the fifth. Do the same thing starting on each degree of the C major scale and you generate the seven triads available in that key: C, E, G then D, F, A then E, G, B and so on up to B, D, F.

Those seven chords are the entire native vocabulary of the key. Almost every song you know in C major is assembled from that set, occasionally borrowing from elsewhere for colour. It is a smaller vocabulary than people expect.

The four triad types

Every triad is defined by the size of its two stacked thirds. A major third is four semitones; a minor third is three.

A major triad is a major third with a minor third above it. C, E, G: C to E is four semitones, E to G is three. The outer interval is a perfect fifth, seven semitones. Bright, stable, the default.

A minor triad reverses the order: minor third then major third. C, E flat, G. The outer interval is still a perfect fifth, so the chord is equally stable, but the lowered third changes the colour entirely.

A diminished triad stacks two minor thirds. C, E flat, G flat. The outer interval is now only six semitones, a diminished fifth, and the chord sounds compressed and unsettled. It wants to move.

An augmented triad stacks two major thirds. C, E, G sharp. The outer interval is eight semitones, an augmented fifth. Because it divides the octave into three equal parts it sounds suspended and directionless, which is exactly why composers reach for it when they want to float free of a key for a moment.

Adding the seventh

Stack one more third on top of a triad and you have a seventh chord, four notes instead of three. The additions are not interchangeable, and the common types each have a distinct job.

A dominant seventh is a major triad plus a minor seventh, ten semitones above the root: C, E, G, B flat. It contains a diminished fifth between E and B flat, and that instability is the whole point. A dominant seventh built on the fifth degree of a key pulls strongly home. In C major, the chord G, B, D, F resolves to C with real force. Almost all functional harmony from Bach to blues runs on this mechanism.

A major seventh is a major triad plus a major seventh, eleven semitones: C, E, G, B. The top note sits a semitone below the octave, producing that soft, hovering sound associated with jazz ballads and bossa nova. It is restful rather than driving.

A minor seventh is a minor triad plus a minor seventh: C, E flat, G, B flat. Mild, mellow, and the backbone of the two-five-one progression. A half-diminished seventh is a diminished triad plus a minor seventh, C, E flat, G flat, B flat, common in minor keys. A fully diminished seventh stacks a further minor third, C, E flat, G flat, B double flat, giving four notes equally spaced three semitones apart, a chord that can pivot to almost any key and has been getting composers out of trouble for two hundred years.

Inversions and voicing

A chord is not required to sit with its root at the bottom. Take C, E, G and move the C up an octave and you get E, G, C: the same chord in first inversion. Move the E as well and you get G, C, E, second inversion. The identity is unchanged; the colour and the smoothness of the bass line are not.

This is what bass players are doing when a chart says C with a slash and an E underneath. The harmony is C major, but the bass is asked to play E, usually so that the bass line steps rather than leaps. Voicing decisions like this are most of what separates a musical accompaniment from a correct one.

Chords under the hands

The Stradella bass system on an accordion is a chord machine, and it makes the theory visible. The chord rows give you preset major, minor, seventh and diminished buttons for every root, laid out so that the tonic, subdominant and dominant of a key sit in a compact cluster: C major with F major one button one way and G major one button the other. Those three chords plus the relative minor cover a staggering proportion of popular repertoire.

The system has honest limits. The presets are fixed voicings, they usually omit the fifth from seventh chords, and you cannot invert them or choose which note is on top. For most accompaniment this is irrelevant. For anything contrapuntal it is fatal, which is why free-bass and converter instruments exist.

Double bass players meet chords from underneath. You rarely play all the notes; you play the one that defines the chord's position and let the rest of the ensemble supply the colour. Knowing that the seventh of a dominant chord wants to fall by a step, and the third wants to rise, tells you what the harmony will do next before anyone says so. In ensemble classes at A&G Academy that single habit is what turns a bass line from correct into inevitable.

Frequently asked

What is the difference between a triad and a chord?

A triad is a specific kind of chord: three notes built by stacking two thirds. Chord is the general word for any group of notes sounded together as a harmony, including four-note sevenths, five-note ninths, and voicings that omit or double notes. Every triad is a chord; not every chord is a triad.

Why is it called a dominant seventh?

Because it is built on the fifth degree of the scale, which is traditionally named the dominant. In C major that degree is G, and the chord G, B, D, F is the dominant seventh. The name describes its position and function in the key, not its sound in isolation, which is why the label survives even in pop and blues.

Do I need to learn chords to play classical music?

You will play better if you do. Written-out classical parts do not require you to build chords yourself, but recognising that a passage is an inverted dominant seventh tells you where it is heading, how to shape the phrase, and which notes to lean on. It converts reading notes into understanding music.


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