My understanding of chord building must be wrong.

If I read a chord is dim I understood it meant that I needed to take the chord and flatten the fifth, so I would look for my root, third and fifth from the major scale and flatten thr fifth.

Apparently, I'm wrong. Why? What's the chord I thought was dim called?

  • As a mnemonic: you can think of augmented as "major, but made bigger" (one 3rd is already major, so you widen the minor third so that they're both major) and diminished is "minor, but made even smaller," so that you take a minor triad and make its major third into another minor. Commented May 17 at 13:02
  • Not just C# dim, but every other dim chord. I guess you mean why does a dim chord have dim5, but needs m3between root and ^3?
    – Tim
    Commented May 17 at 15:47

6 Answers 6


The easiest way to memorize (and visualize on the keyboard or fretboard) the basic chord buildup is seeing them as stacked intervals of third:

  • minor 3rd then minor 3rd - diminished
  • major 3rd then minor 3rd - major
  • minor 3rd then major 3rd - minor
  • major 3rd then major 3rd - augmented

note that these are all possible way you can stack minor and major thirds on top of each other and these four options happen to be the four basic chord flavours in western harmony. More generally in classical harmony the chords are build by stacking thirds. Keep stacking thirds on top of these basic triads and you will get 7th, 9th, 11th and 13th chords.

As for the major chord with flattened fifth in most practical cases it is better to think about it as major triad with augmented fourth and it would work great as a tonic chord in lydian mode.

Finally the theory of the chord construction is more of an observation that a recipe so don't stress about it too much :)

  • The series of thirds for identifying and naming triads is useful and is a pretty common way to explain them but I much prefer to think of triads as a type of third plus a type of fifth. That way the root of the chord is always in the mix. Commented May 17 at 20:41
  • I'm really confused by have two thirds, where does the second third come from?
    – Three Diag
    Commented May 17 at 22:40
  • 1
    @ThreeDiag C major chord = (interval from C to E) + (interval from E to G). Commented May 18 at 3:07
  • I'm learning so much from this q, eg the definition of a third. Always thought third meant third in SOME scale, but no ahha
    – Three Diag
    Commented May 18 at 10:21
  • @ThreeDiag it still does kind of mean a 3rd of the major scale, but not a scale that has anything to do with the composition or key of the song, we just use it as our measuring device / interval ruler to measure intervallic distances! Eg, the distance between the root of the II chord and the root of the IV chord is a b3rd. It provides an easy way to describe musical relationships.
    – OwenM
    Commented May 18 at 15:16

A diminished chord always contains the root, the minor third, and the diminished fifth. That's the definition of "diminished chord".

In some styles there's a tendency to implicitly mean a diminished seventh chord when one says diminished, which also includes the diminished seventh.

So for C#dim, that's C#, E, G and maybe Bb.


...C# dim...so I would look for my root, third and fifth from the major scale and flatten the fifth...

When you say you are starting with a major scale, I assume you mean, in this case, C# major.

Let's look at the diatonic diminished triad in C# major, and then move on to your flattening method.

In C# major, tone C# D# E# F# G# A# B# C#, the triad built on the seventh scale degree, the leading tone degree, will be B# D# F#. The intervals above the root of that chord are a minor third (m3) and a diminished fifth (d5) and that is the intervallic structure of all diminished triads: R m3 d5.

...I understood it meant that I needed to take the chord and flatten the fifth...

At a fundamental level it is not about altering some starting chord. There are two ways to think about it:

  • The triad types are derived from the various diatonic triads of the major/minor keys. Ex. a major triad is the I chord of a major scale, a minor triad is the vi chord of a major scale, and the diminished triad is the viio chord of a major scale. That leaves the augmented triad out as an odd chord, but we won't get into that here. Suffice to say the augmented triad comes from the particulars of minor key harmony.
  • Intervals of a third and fifth above a chord root. Those intervals can be major or minor thirds, with diminished, perfect, and augmented fifths. Notice that there are four basic triad types: major, minor, augmented, and diminished. But that does not cover all possible combinations of m3/M3 plus d5/P5/A5 above a root. Combinations like R m3 A5 or R M3 d5 are not among the four basic triads. Why? Because they are not among the diatonic triads.

Now let's get back to your flattening method. Assuming that you mean to take the I chord of C# major, tones C# E# G#, and alter them to get a diminished triad, we need to work with the second method from above and get the intervallic structure of R m3 d5. The root is C#. The minor third above is E natural. The diminished fifth above is G natural. So, you need to flatten both the third and the fifth.

To be complete, it should be pointed out that C#dim is not diatonic to C# major, which is not a problem per se, it's just a fact, and the diatonic diminished chord of C# major is B#dim, tones B# D# F#. (If you play B#dim on a keyboard it looks enharmonically like Cdim, but rest assured the correct spelling of viio in C# major is B#dim tones B# D# F#.

If that last bit about enharmonic spellings is confusing, I suggest reviewing this stuff from C major. That should avoid a lot of confusing enharmonic issues.


Yes, you were mistaken.

C, E♭, G♭ is Cdim.

C, E, G♭ is C(♭5).

  • 2
    OP was looking for C#dim, not C dim
    – nuggethead
    Commented May 17 at 15:33
  • @nuggethead Same idea.
    – Laurence
    Commented May 17 at 15:43
  • 1
    Yes, but if op is a beginner to this, knowing C dim may or may not help to learn C# dim
    – nuggethead
    Commented May 17 at 16:01
  • This is helpful, seeing the notation I realised I've seen chord (flat 5) while studying bass and thought that's called dim, but it's not.
    – Three Diag
    Commented May 17 at 22:23

The main 4 types of chords.

You have these four chords as a base and then things get added to them like 7ths or 9ths, but these are the base of which all other chords are built.

All chords are a root note with certain set notes a set interval from that root. The notes can be played together or one at a time like a broken chord or arpeggio, for instance.

Major Chords

This consists of a root note, a Major Third, and a perfect Fifth.

Minor Chords

This consists of a root note, minor third, and a perfect Fifth.

Diminished Chords

This consists of a root note, minor third, and a diminished Fifth

Augmented Chords

This consists of a root note, Major Third, and an augmented Fifth.

Think of the Augmented chord as an accordion moving away from the root with the Major Third and Augmented fifth and the Diminished chord as an accordion moving towards the root with the minor third and the diminished fifth.


The third is flattened, as well as the 5th. It is basically stacked minor 3rd intervals (strictly speaking a 4 note chord like C, Eb, F#, A is a dim 6th).

So minor third is always there.


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