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The problem it solves
Triads are the bricks all tonal harmony is built from. Understanding how they stack in thirds opens the door to forming, reading and recognising any basic chord.
Detailed theory
Key idea
A triad has three notes: root, third and fifth (1-3-5).
It is built by stacking two thirds, that is, skipping a letter of the scale each time (C, [D], E, [F], G).
Understand it
Stacking in thirds means starting from a base note and taking every other one: you skip the neighbouring note. From C you take E (skipping D) and from E you take G (skipping F). That gives you C-E-G, a triad.
The three notes have functional names: the base is the root, the middle one the third and the top one the fifth. That is why we speak of the 1-3-5 formula: they are the scale degrees they occupy.
What gives the triad its character is how the two stacked thirds are. A third can be major (four semitones) or minor (three semitones), and the combination chooses the quality of the chord.
Major below and minor above makes a major triad (C-E-G); minor below and major above makes a minor triad (C-Eb-G); two minor thirds make a diminished one; two major thirds, an augmented one. The distance between root and fifth changes too: a perfect fifth in the major and minor, a diminished fifth in the diminished, an augmented fifth in the augmented.
An analogy: building a triad is like stacking blocks skipping one each time (1 → 3 → 5). Depending on which blocks you use, the tower stands firmer or more tense, but it is always three pieces stacked the same way.
Stacked triad
C minor: the same root and fifth, but the third drops to Eb. Seeing the two triads side by side shows how the third chooses the quality.
How to recognise it
How it's written
It is written by stacking three alternate notes over the root: 1-3-5, for example C-E-G. On the staff they look like three notes one above the other, all on lines or all in spaces.
How it feels
It sounds like a compact block of three notes held together. Play it as an arpeggio and you hear the three pitches separately; play it all at once and you hear a single chord colour.
Common mistake
Thinking that three consecutive notes (like C-D-E) are already a triad: you must stack thirds, skipping a letter, not take neighbouring notes.
Playing the three notes as a block without knowing which is the root, the third and the fifth.
Try it
From C, skip a letter each time to build the triad: C, skip D, E, skip F, G. You get C-E-G.
Try the same from A (A-C-E) and from D (D-F-A): each time you stack two thirds over the root.
On the instrument
Stacked triad
C major: a triad stacks C, E and G, skipping a letter each time (C, [D], E, [F], G).
Where it's used
- Building chords
- Stacking two thirds over any note to form its triad (1-3-5).
- Reading verticalities
- Recognising three stacked notes on the staff as a triad and naming its root, third and fifth.
- Understanding quality
- Working out whether a chord is major, minor or diminished from how the two stacked thirds are.
Examples
Stacked triad
The triad D-F-A: stacks two thirds over D, skipping E and skipping G. Play it as an arpeggio and as a chord.
Mini test
Check that you've got it.
0/10 answeredQuestion 1/10
How many notes does a triad have and how are they related?