Lesson 5: All About Triads

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Key Takeaways

• A triad's root never changes; the bass (lowest sounding note) determines inversion (root position / first / second)
• In any major key: I, IV, V = major; ii, iii, vi = minor; vii° = diminished — these qualities are automatic
• Minor keys add complexity: V is almost always raised to major; vii appears as both a subtonic triad (♮VII, major) and a leading-tone triad (vii°, diminished)

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The Four Triad Types ▶ 0:42

Quality	Bottom 3rd	Top 3rd	Bounding 5th	Consonant?
Diminished	minor	minor	diminished	✗
Minor	minor	major	perfect	✓
Major	major	minor	perfect	✓
Augmented	major	major	augmented	✗

• Each successive type is built by raising the top note of the previous chord by one semitone
• Major/minor are acoustically consonant (perfect 5th); diminished/augmented are dissonant and rarer

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Chord Tones, Voicing & Position ▶ 3:33

• Root = note the chord is named after; chordal third = a 3rd above root; chordal fifth = a 5th above root
• A triad is defined by its three pitch classes — doublings and register don't change its identity
• Open position: gaps exist between notes where another chord tone could fit; closed position: no such gaps (notes packed as tightly as possible)

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How to Identify a Mystery Triad ▶ 7:00

1. Identify all notes (ignore doublings)
2. Stack as thirds — try all three orderings in closed position; whichever gives a stack of thirds has the root on bottom
3. Determine quality — check the bounding fifth (dim/perf/aug), then the bottom third (minor → minor triad; major → major triad)

Example analyzed at ▶ 7:00: chord with A, F♯, C♯ → F♯ minor

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Diatonic Triads in Major Keys ▶ 8:35

Scale Degrees	Quality	Roman Numeral Style
I, IV, V	Major	Uppercase (I, IV, V)
ii, iii, vi	Minor	Lowercase (ii, iii, vi)
vii	Diminished	Lowercase + ° (vii°)

• Functional names (tonic, supertonic, mediant, etc.) apply equally to chords and scale degrees

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Diatonic Triads in Minor Keys ▶ 11:01

• Natural minor defaults: i, iv, v = minor; III, VI, VII = major; ii° = diminished
• Harmonic minor raises ♯7: turns v → V (major dominant — nearly universal in classical style); turns ♮VII → vii° (leading-tone triad, diminished)
• Melodic minor raises ♯6: turns iv → IV (major subdominant — less common)
• Subtonic triad (♮VII, major) vs. leading-tone triad (vii°, diminished) — different roots, different Roman numerals

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Inversions ▶ 15:46

Inversion	Bass note	Name
Root position	Root	—
First inversion	Chordal 3rd	1st inv.
Second inversion	Chordal 5th	2nd inv.

• Root ≠ bass: root is an abstract property of the chord; bass is whichever note is lowest in a specific voicing
• Upper voice arrangement is irrelevant to inversion — only the lowest note matters
• Figured bass notation (6, 6/4) for inversions introduced in Video 7