Chord Structure · Lesson 1
The Four Kinds of Triad
about 14 min
What this lesson is for
A triad is two thirds stacked on top of each other. Because a third is either major or minor, only four combinations exist, and those four account for every major, minor, diminished, and augmented triad. This lesson also shows how taking every other note of a scale produces chords for free.
- Explain that a triad consists of a root, a third, and a fifth
- Derive the four triad types from the order in which major and minor thirds are stacked
- Explain how taking every other note of a scale produces a triad
- Explain the instability of diminished and augmented triads in terms of the interval to the fifth
In this lesson
This unit is about chords. A chord is several notes sounding together, but Western harmony builds them on one explicit principle: stacking thirds.
The most basic chord, made of three notes, is a triad.
Two thirds, stacked
A triad is built by stacking two thirds above a root. Each of the three notes has a name.
| Note | Position | Interval from the root |
|---|---|---|
| root | bottom | — |
| third | a third above | major 3rd or minor 3rd |
| fifth | another third above | perfect, diminished, or augmented 5th |
The constraint “two thirds” is doing real work here. A third is either major or minor, so there are only 2 × 2 = 4 possible combinations. Those four combinations are the four triad types, and no fifth kind is possible.
There are exactly four
| Type | Lower third | Upper third | Root to fifth | Symbol |
|---|---|---|---|---|
| Major | major | minor | perfect 5th | C |
| Minor | minor | major | perfect 5th | Cm |
| Diminished | minor | minor | diminished 5th | Cdim |
| Augmented | major | major | augmented 5th | Caug |
The column worth studying is the fifth. Major and minor triads both put a perfect 5th between root and fifth. Since the previous unit established the perfect 5th as a perfect consonance, both of those chords have a stable outer frame.
Diminished and augmented triads have a diminished and an augmented 5th respectively, and both of those are dissonant. Which is why neither chord can settle: each one is under pressure to move on.
The four types side by side
Check: The root is C throughout. The first two have a perfect 5th holding them together and sound settled. In the third and fourth the fifth has been altered, and both hang in the air, audibly wanting to go somewhere.
major + minor 3rd / perfect 5th
Check
Which note differs between a major and a minor triad?
What a triad looks like on the staff
Written on the staff, a triad takes a distinctive shape, and recognizing it lets you identify a chord at a glance.
A third means skipping one note of the scale, which on the staff is exactly one position — line to the next line, or space to the next space. Stacking two thirds means making that one-position move twice.
So a triad in root position always looks like one of two things:
- all three notes on lines (E–G–B, for instance)
- all three notes in spaces (F–A–C)
Three notes packed evenly with no gaps means root position. A gap anywhere means the notes have been rearranged — an inversion, which the next lesson covers.
Being able to see this rather than compute it is a genuinely useful reading shortcut. You do not have to name each note and calculate intervals to know what kind of stacking you are looking at.
Check
Three notes are stacked evenly on the staff with no gaps. What can you conclude with certainty?
Take every other note of the scale
You do not have to count thirds each time. Taking every other note of a scale stacks two thirds automatically.
From the C major scale C D E F G A B, take C, E, G. Then D, F, A. Then E, G, B. Doing this on all seven degrees produces seven triads.
Something interesting happens here. The same operation yields different types of chord.
- C, F, G → major triads
- Dm, Em, Am → minor triads
- Bdim → a diminished triad
The cause is the two half steps inside the scale. A third taken by skipping one note is minor where it spans a half step and major where it does not. The placement of the half steps in the scale determines the sequence of chord types.
That is the underlying reality of the diatonic chords covered next. Every major key produces the identical sequence: major, minor, minor, major, major, minor, diminished.
Seven chords for free, just by stacking the scale
Check: These are the seven chords produced by taking every other note of C major. Follow the pattern by ear — major, minor, minor, major, major, minor, diminished. The last one is audibly unstable, and the second example confirms where it wants to go.
maj min min maj maj min dim
Check
What triad stands on D in C major?
Where augmented triads come from
You may have noticed that Fig. 4 contains no augmented triad. Augmented triads never appear among the diatonic triads of a major scale.
The place they arise naturally is harmonic minor. Raising the seventh degree, as the previous unit did, changes the chord built on the third degree.
In C harmonic minor, stacking E♭, G, B gives a major 3rd from E♭ to G and another major 3rd from G to B, which makes an augmented triad. With the natural minor’s B♭ it would have been a plain major triad.
So the augmented triad exists as a side effect of raising the leading tone. It is far rarer than the other three types, and it is generally used in passing rather than carrying a function of its own.
Check
On which degree of which scale does an augmented triad appear diatonically?
Reading chord symbols
The labels used above — C, Cm, Cdim, Caug — are chord symbols, and the convention is simple:
- a capital letter name = the root
- whatever follows = the type (nothing at all means a major triad)
Major triads take no suffix because they are treated as the default case. Popular music uses m, dim, and aug; classical contexts use a different system entirely, based on Roman numerals. How the two notations correspond is the subject of the final lesson in this unit.
Cycle through the four types in the chord finder Fix the root at C and switch between major, minor, dim, and aug. Watch the keyboard and confirm that only the third and the fifth ever move Chord Finder → Look ahead at the diatonic chords All seven chords of C major appear together. Change the key and confirm that the sequence major-minor-minor-major-major-minor-diminished is preserved Diatonic Chord Reference →Exercises
Every answer here is cross-checked against the theory engine. Work them out before you grade.
- 1
Spell the C major triad from the bottom up, using single spaces between names.
- 2
Spell the A minor triad from the bottom up, using single spaces between names.
- 3
Spell the B diminished triad from the bottom up, using single spaces between names.
- 4
What is the interval from the root to the fifth of a diminished triad?
- 5
How do you extract a triad from a scale?
Before you move on
Check that you can explain each point above in your own words.