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Why do diminished chords have 3rd minor , not 3rd diminished ?

and what is the name of this chord : root + 3rd diminished + 5th diminished?

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  • 1
    Good point. Diminished seventh chord has a dim7, basic dim triad has dim5. Why should it have m3..? Slight misnomer, like minor 6th chord?
    – Tim
    Jun 3, 2020 at 10:54
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    The chord with a diminished third and a diminished fifth is called a double-diminished triad. That question was answered here.
    – Matt L.
    Jun 3, 2020 at 14:33
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    @MichaelCurtis: A double-diminished chord is defined only by the intervals between the root and the other two notes, just like major and minor triads. Any triad with a diminished third and a diminished fifth is a double-diminished triad.
    – Matt L.
    Jun 3, 2020 at 15:02
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    Lots of answers describing the intervals in a diminished chord but not many explicitly answering the question of why those intervals are given that name. Jun 3, 2020 at 16:26
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    And a corrollary - why has an augmented chord got M3?
    – Tim
    Jun 3, 2020 at 16:48

8 Answers 8

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In a diminished triad the diminished refers to the fifth of the chord and the third in understood to be a minor third.

In a diminished seventh chord the diminished refers to the seventh of the chord and the third is understood to be a minor third and the fifth is understood to be diminished.

The names are not a complete list of the intervals of the chords.

Think of it like the name major scale, not all the intervals in that scale are major, it's just a name, you still need to learn the interval structure.

To some degree the simple names work because the interval structure is understood relative to the diatonic scale. Major/minor triad refers to the third only and the fifth is assumed perfect, because all fifths are perfect except the one above the leading tone, and so the triad built on the leading tone is called diminished referring to the unique fifth in only that one diatonic triad.

In the case of non-diatonic seventh chords, the naming becomes more explicit, sort of listing out intervals. For example a minor major seventh chord is a minor triad with a major seventh added.

Why do diminished chords have 3rd minor?

Because the thirds of the triads built on the leading tone of diatonic scales, or the second degree on minor diatonic scales, are minor thirds.

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  • In key C, there can often be found the chord Cdim. C Eb, Gb. That's based on the root, not any leading note,. How does that fit with your last para?
    – Tim
    Jun 4, 2020 at 7:47
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    First, the basic definition of a diminished chord IS the triad built on the leading tone. You can then use the same interval structure rooted somewhere that won't produce a diatonic chord. In your example, the chord in a diatonic setting would be to a tonic of Db. In the key of C it's not diatonic. You would probably identify that chord as a common tone diminished chord, but like all harmony/spelling examples it depends where it goes next. Notice how the name becomes much more descriptive when it's not diatonic. Jun 4, 2020 at 13:37
  • We may have to agree to differ, but the chord that's based on the leading note is called 'the chord of the diminished seventh' of that key. That in itself isn't (in my opinion) enough to reason that diminished chords must have m3. In fact, apart from 'the chord of the diminished seventh is generally anything but diatonic. And what's weirder, in key C, it's based on note B, which strangely, isn't even the diminished seventh note! Curiouser and curiouser...
    – Tim
    Jun 4, 2020 at 13:55
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    The diatonic triad based on the leading tone is diminished. In what major/minor key is that not true? And diminished in its name refers to the fifth. The diatonic third above the leading tone - in major/minor keys - is always a minor third, in what key is that not true? Jun 4, 2020 at 17:15
  • Using key signatures as the definition of diatonic, a diminished seventh is always diatonic. You can't spell that interval in a major/minor key signature without using an accidental. But that point is only about the seventh. It has no bearing on the third or fifth - it doesn't answer the original question. I brought up seventh chords in my answer only to compare the more explicit naming used for the non-diatonic types. Jun 4, 2020 at 17:22
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A diminished chord has a diminished fifth, not diminished third https://en.wikipedia.org/wiki/Diminished_triad

Diminished third is two semitones https://en.wikipedia.org/wiki/Diminished_third

"root + 3rd diminished + 5th diminished" ... if you play that rooted on the first scale degree, it gives a Lydian'ish sound, because it's enharmonic to something like II 7 / I.

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Let's say the root is F#, for example. The diminished chord would be F# A C. (If it were a diminished 7th chord, it would be F# A C Eb.)

The diminished third would be Ab. That would make the chord F# Ab C (plus Eb if you add the diminished seventh). That chord exists: it's the German sixth chord.

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  • We don't need to add the diminished seventh to make it an "existing" chord: the chord F# Ab C in first inversion (Ab C F#) is just an Italian sixth chord.
    – Matt L.
    Jun 3, 2020 at 15:16
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Short answer - due to enharmonic equivalence in classical, tertian harmony.

Short answer 2 - the name of it is "enharmonic notation of the 3rd inversion of D7 without the 5th"

A minor third is unambiguous, and generally accepted to be consonant ("pleasant sounding").

By using a diminished third, we are dangerously closing in to a non-third-based chord, which are generally regarded as too dissonant for basic harmony (i.e. a different area or level of expertise).

A diminished third is more similar to a "second" interval.

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    While I think your response is a good one it seems like the question is more about etymology. Why the name, not the structure. The Dim7 chord can be thought of as a Dom7 chord with the (3, 5, b7) flattened in one fell swoop. Whereas there is such a thing as a Dim 3rd and the OP seems to wonder why not diminish everything.
    – user50691
    Jun 3, 2020 at 14:03
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If we define the triads of all degrees of any scale (minor or major) we have major thirds and minor thirds with perfect fifths: the terms major / minor refer to the lower third.

Only the seventh degree ti-re-fa (= B,D,F) in a major scale and the second degree of a minor scale se-ti-fa (= G#,B,D) has a diminished fifth - that's why this chords are named "diminished": the term diminished refers to the dim. 5th.

(In a chromatic minor scale the 3rd degree can contain 2 major thirds (do-mi-se = C,E,G#). This triad is named "augmented" referring to the augmented fifth (+5 or #5).

"Double diminished" would mean that both intervals (3rd and 5th) are diminished: double diminished refers to the third and the fifth! (so double means that both intervals are diminished).

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  • I think OP is questioning why m3 in diminished, whereas there is M3 in augmented (that part is mine).
    – Tim
    Jun 3, 2020 at 16:47
  • I've tried to explain where term diminished in triads comes from. The thirds are minor or major, * diminished* and augmented refer to the 5th. Jun 3, 2020 at 17:05
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There isn't always a completely logical reason why things are named as they are.

A major scale contains both major and minor intervals. Likewise a minor scale.

A diminished triad has minor 3rd and diminished 5th

A diminished 7th chord has minor 3rd, diminished 5th and diminished 7th.

I suggest you accept this and move on!

A diminished 3rd is theoretically possible. (So is an augmented 7th.) But they sound so like a major 2nd (and an octave) that there's rarely any point in spelling them that way. And I don't think you'll gain anything from worrying about such extreme cases.

Root, diminished 3rd and diminished 5th (C, E♭♭, G♭) will be heard as C, D, F♯, a bare-bones D7.

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  • Perhaps the person who disagrees could say why?
    – Laurence
    Jun 3, 2020 at 23:36
  • It seems good to me so I balanced the down vote with an up.
    – badjohn
    Jun 4, 2020 at 12:58
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A diminished triad consists of two minor thirds. The scale degrees would be 1 b3 b5.

A triad with a diminished triad and a diminished fifth would contain the notes A, Cb and Eb. It will actually sound like B D# A which is a B7 chord without the fifth.

The diminished triad is the seventh diatonic triad in the major scale. For instance, the viidim in the G major scale would be F# A C (all the notes are in the G major scale).

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I will answer your question from a Functional Harmonic perspective and I will only look at your first question: why do diminished chords have a minor third.

As has been mentioned, the diminished aspect of diminished chords (diminished triads in particular) refers to the fifth of the chord, for example B-D-F where the B to F is a diminished fifth. You are asking specifically about the third of the chord, the B to F. Why is it a minor third?

In fact it is not a minor third...atleast not in the sense of this being a type of B-minor triad. The B-diminished chord is not actually related to the B-minor triad but rather is related to G dominant 7 (G7). The B-D-F is the "upper part" of a G-B-D-F and therefore the mode of a diminished triad is actually major. The foundational chord of B-dim is G7 and so the modal quality come from the G-B dyad, a major third. In functional harmony B-dim is a G7 chord with omitted root, that is, an omitted G.

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