I've started learning music theory bit by bit but there's something that I can't quite figure out.

I know that when you're in a certain key, the first is major, the second and third are minor, the fourth and fifth are major, the sixth is minor and the 7th is diminished.

What I don't understand is why.


What I understand from the comments and sources is that the intervals between the triads make up whether a chord is major, minor or diminished.

Key of C:

C . . . E . . G --> (major)
D . . F . . . A --> (minor)
E . . G . . . B --> (minor)
F . . . A . . C --> (major)
G . . . B . . D --> (major)
A . . C . . . E --> (minor)
B . . D . . F   --> (diminished)

How will this play out for let's say... the key of D?


4 Answers 4


We call these triad qualities.

Whenever you determine the quality of a triad, you have to base it off of the root of the triad, not the tonic of the overall key.

So in C major, you have D F A. Even though you're in C major (we call C the tonic of the key), we need to think briefly in the key of D to determine the quality of the D F A triad.

In D major, there are two sharps: F# and C#. Since the F in D major is actually F♯, F♯ is the major third above D. As such, a D major triad would be D F♯ A. Since we want D F A, we see that the major third F♯ is lowered a half step to the minor third F♮.

So we have D F, which is a minor third. Now we need to look at the fifth D A.

But there's a nice trick with fifths. If two pitches are a perfect fifth apart, they will share the same accidental. The only exception are the pairs B♭/F and B/F♯. Otherwise, all pitches that are a fifth apart and have the same accidental create a perfect fifth. D A matches this rule, so it's a perfect fifth.

Now we have D F A, which has a boundary interval of a perfect fifth and a third that is minor. This creates a minor triad.

If we turn now to F A C, we think again in the key of the triad's root: here, F. F has one flat (B♭) in the key signature, otherwise everything else is natural in the F major key. Therefore, A is a major third above F, and we know from our previous rule that F C is a perfect fifth. As such, a perfect fifth with a major third inside is a major triad.

There are two other triad qualities: diminished and augmented. Diminished triads have a diminished fifth (a perfect fifth that's one half step too small) with a minor third, and augmented triads have an augmented fifth (a perfect fifth that's one half step too large) with a major third.

In summary:

  • Major triad: major third with a perfect fifth; scale degrees 1 3 5 of a major scale.
  • Minor triad: minor third with a perfect fifth; scale degrees 1 ♭3 5 of a major scale, or scale degrees 1 3 5 of a minor scale.
  • Diminished triad: minor third with a diminished fifth; scale degrees 1 ♭3 ♭5 of a major scale.
  • Augmented triad: major third with an augmented fifth; scale degrees 1 3 ♯5 of a major scale.

Lastly, the pattern of major and minor triads (with the last one being diminished) is what we call diatonic triads. This means that this is the pattern of chord qualities that obtains in any major key without any chromatic alterations.

As such, no matter what major key you're in, the chord built on the third scale degree of a major key will always be minor as long as there aren't any chromatic pitches added in. Similarly, the chord built on the fifth scale degree will always be major.

Note that this pattern of diatonic triads is different in a minor key. For this issue, see Understanding minor key harmony


Starting from one note, let's say C.

The major part is the 3rd note on, E. It's an interval of a major third.

The minor part is a flat 3rd on, Eb. It's an interval of a minor third.

The other note in both cases is P5 - a perfect fifth.

Onto diminished. There's a m3 (Eb) and a dim5 (Gb), making the Co spelled C Eb Gb.

Now you've added to the question - a different explanation is needed.

In D major, the pattern of diatonic chords is the same pattern as in C, or any of the other ten major keys. Maj, min, min, maj, maj, min, dim. So, triads - on D: D F# A (maj) on E: E B G (min), on F#: F# A C# (min), on G: G B D (Maj), on A: A C# E (maj), on B: B D F# (min), on C#: C# E G (dim).

Note - each triad is made up from notes with letter names a third apart, but all match the criteria for maj., min., or dim.


Triads are a set of three notes, set intervals away from a root note, the intervals are as follows.

Major Chords are chords with a root note, a Major third above it, and a perfect fifth above the root note

Minor Chords are chords with a root note, minor third above it, and also a perfect fifth above the root.

Augmented chords are chords with Major thirds and augmented fifths

Diminshed chords are chords with a minor third and a diminished fifth

These types of chords are key agnostic, yes you get certain scale degrees where certain chords fit naturally but there are massive amounts of music that are only loosely adhering to a key that can have any manner of chords on the various scale degrees of the scale.

You need to know how to figure them out, not just learn what chords fit at which scale degree.


How will this play out for let's say... the key of D?

The notes in D major are: D, E, F♯, G, A, B, and C♯.

The pattern of chords diatonic to the key will be exactly the same as for C (for which the chords are enumerated in the question linked to above), but a tone higher.

From www.piano-keyboard-guide.com/key-of-d.html :

From www.piano-keyboard-guide.com/key-of-d.html

And Neil Meyer's point is worth repeating - just because a piece is 'in D Major', it doesn't necessarily mean that these are the only chords you'll find in the piece!

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