Topic 3.1 Triads: build a triad as three pitches stacked in thirds (root, third, fifth), and identify its quality as major, minor, diminished or augmented.

What this topic is asking

The College Board (Topic 3.1) wants you to build a triad as three pitches stacked in thirds (a root, a third and a fifth) and to identify its quality as major, minor, diminished or augmented from the size of the third and the fifth above the root. You also need to recognize the diatonic triads that occur naturally on each degree of a major or minor scale.

Stacking thirds

Because the notes are stacked in thirds, a triad on the staff appears as three notes either all on lines or all in spaces. To spell a triad correctly, start on the root letter, skip a letter to find the third, then skip another letter to find the fifth (C skip D to E skip F to G).

The four triad qualities

The quality of a triad is fixed by the size of its two stacked thirds, which in turn fixes the fifth:

• Major triad: major third (four half steps) on the bottom, minor third on top; root to fifth is a perfect fifth. Example C, E, G.
• Minor triad: minor third (three half steps) on the bottom, major third on top; root to fifth is a perfect fifth. Example C, E flat, G.
• Diminished triad: two minor thirds; root to fifth is a diminished fifth (six half steps). Example C, E flat, G flat.
• Augmented triad: two major thirds; root to fifth is an augmented fifth (eight half steps). Example C, E, G sharp.

Diatonic triads of a key

When you build a triad on each degree of a scale using only the notes of that key, the qualities follow a fixed pattern. In a major key the pattern of qualities from degree 1 to 7 is major, minor, minor, major, major, minor, diminished. In natural minor it is minor, diminished, major, minor, minor, major, major. These diatonic qualities are the foundation of Roman numeral analysis: an uppercase numeral marks a major triad, lowercase marks minor, lowercase with a small circle marks diminished, and uppercase with a plus marks augmented.

Why quality, not pitch, defines a chord

The central idea is that a triad's identity is its interval shape, not its absolute pitches. C major and G major share no common spelling, yet both are major triads because both stack a major third under a minor third. This is why you can transpose a whole progression and keep its harmonic meaning: every chord keeps its quality even though every pitch changes. It also explains why spelling matters. A C augmented triad must be written C, E, G sharp and not C, E, A flat, because the chord is a stack of thirds (C to E to G), and G sharp keeps the top note a true third above E. Writing A flat would make the top interval a fourth on paper, hiding the triad. Hearing and labelling chords by quality, rather than by the notes that happen to be present, is the skill the whole harmony sequence builds on.

Building a triad by hand

To construct a triad, write the root, then the letter a third above, then the letter a third above that, giving three alternate letters. Now adjust accidentals so the lower third matches the quality you want (four half steps for major, three for minor) and check the resulting fifth.

Try this

Q1. What interval lies between the root and the third of a minor triad? [1 point]

• Cue. A minor third (three half steps); the major third on top completes the perfect fifth.

Q2. Build the diminished triad on B using only natural notes and name its three pitches. [2 points]

• Cue. B, D, F: B to D is a minor third, D to F is a minor third, and B to F is a diminished fifth, so the triad is diminished.