Triads & Seventh Chords

You’ll be able to

Build major, minor, diminished, and augmented triads from stacked thirds
Identify a chord’s root, third, and fifth in any inversion
Construct and name the five common seventh-chord qualities

Triads: four qualities from stacked thirds

A triad is three pitches stacked in thirds: a root, a third above it, and a fifth above the root. The two thirds’ qualities set the triad’s quality. Major = major third + minor third (C–E–G). Minor = minor third + major third (C–E♭–G, or A–C–E). Diminished = two minor thirds, giving a diminished fifth (B–D–F). Augmented = two major thirds, giving an augmented fifth (C–E–G♯). Major and minor triads are consonant; diminished and augmented are dissonant.

Inversions and figured bass

The bass note (lowest) determines a chord’s inversion. With the root in the bass it is root position (figured 5/3, usually left blank). With the third in the bass it is first inversion, figured 6. With the fifth in the bass it is second inversion, figured 6/4. The chord keeps its identity and root through all inversions — a C major triad is still C–E–G whether voiced C–E–G, E–G–C, or G–C–E. Roman numerals add the figures: I⁶ is a first-inversion tonic triad.

Seventh chords: five qualities

Add another third on top of a triad and you get a seventh chord, named by the triad plus the seventh above the root. The five common types: major seventh (major triad + M7, C–E–G–B), dominant seventh (major triad + m7, G–B–D–F), minor seventh (minor triad + m7, D–F–A–C), half-diminished seventh (diminished triad + m7, B–D–F–A, written ø7), and fully diminished seventh (diminished triad + d7, B–D–F–A♭, written °7). The dominant seventh — with its built-in tritone between the 3rd and 7th — is the engine of tonal cadences.

Chord-quality recipes

Major=M3+m3 · minor=m3+M3 · dim=m3+m3 · aug=M3+M3 || seventh chord = triad + (M7, m7, or d7)

Read a triad from its two stacked thirds; add the top third to get the seventh. Figured bass for triads: root=(blank), first inv=6, second inv=6/4.

Worked example

Identify the quality of the chord spelled G–B–D–F.

1.Stack the notes in thirds and check the triad G–B–D: G to B is a major third, B to D is a minor third — a major triad.
2.Now the seventh: from the root G up to F is 10 half steps, a minor seventh.
3.A major triad plus a minor seventh is a dominant seventh chord.
4.Confirm the signature tritone between the third (B) and seventh (F): B up to F is a diminished fifth — the hallmark of a dominant seventh.

Answer: G–B–D–F is a dominant seventh chord (G7): a major triad (G–B–D) with an added minor seventh (F), containing the B–F tritone.

Checkpoint

Which spelling is a D minor triad?

Tip

When identifying a chord from scattered notes, first stack it in thirds to find the root, then measure the two thirds. If the notes are voiced out of order (e.g. E–C–G), rearrange to close position (C–E–G) before naming the quality and inversion.

Checkpoint

What quality of seventh chord is spelled B–D–F–A?

On the exam

Distinguish the two diminished sevenths by the top interval: half-diminished (ø7) has a minor seventh, fully diminished (°7) has a diminished seventh (a stack of three minor thirds). In major keys the viiø7 is common; the fully diminished °7 usually signals a minor key or a secondary function.

Answer the 2 checkpoints as you read.