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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?

  • 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 at 10:54
  • 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 at 14:33
  • @Matt but what if the root isn't altered? like C Ebb Gb rather than C# Eb G – Michael Curtis Jun 3 at 14:59
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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. – dissemin8or Jun 3 at 16:26
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    And a corrollary - why has an augmented chord got M3? – Tim Jun 3 at 16:48
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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 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. – Michael Curtis Jun 4 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 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? – Michael Curtis Jun 4 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. – Michael Curtis Jun 4 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 at 15:16
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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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  • 5
    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. – ggcg Jun 3 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 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. – Albrecht Hügli Jun 3 at 17:05
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There isn't always a completely logical reason why things are named as they are.

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 Payne Jun 3 at 23:36
  • It seems good to me so I balanced the down vote with an up. – badjohn Jun 4 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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Basically there are two types of diminished chords, namely, full diminished and half diminished chords. A full diminished chord is also called a diminished seventh(dim7) chord. A half diminished chord is called a minor seventh flat five(min7b5) chord.

A diminished triad has three equally separated notes of a diatonic scale, the first, flatted third and flatted fifth (1, b3, b5). This is diminished triad. This answers your first question i.e. why does a diminished chord has a minor third and not a diminished third? Note that the diminished triad has three notes which are 1, b3, b5. The b3 is the minor 3rd. The minor third note is made by flatting a major third. So a diminished chord has a minor third and not a diminished third because the diminished triad has a b3 note which is nothing but a minor third.

Also, a diminished third is a flatted minor third. It is the note one semitone behind a minor third. For example, in the diatonic scale of A, the third note is C# which is a major third. Flatting it would take us to C which is a minor third. Again flatting it would take us to Cb or B which is a diminished third.

A full diminished chord is made up by adding a doubly flatted seventh note. So, a full diminished chord has following notes:

  • 1
  • b3
  • b5
  • bb7

Note that the last note(bb7) is not a part of the diatonic scale. For example, the scale of A consists of the following notes:

A B C# D E F# G# A

The first note is A. The flatted third would be C. A flatted fifth would be Eb and the double flatted seventh would be F#. All these notes would make up a full diminished chord. However, to construct a half diminished chord, the notes used will be, A, C, Eb and G. Note that only the last note is different in both the chords.

A full diminished chord is not played as the incomplete tonic parallel chord in a diatonic scale because the double flatted seventh note is not a part of the diatonic scale. What makes the incomplete tonic parallel or the seventh chord of a major chord progression is the half diminished chord or the m7b5 chord because all of its notes are a part of the diatonic scale. For example, in the chord progression of A major scale the seventh chord would be a G#dim (G#min7b5) which contains the notes G#, B, D and F#. These notes are part of the a major scale. A G#dim7 i.e. G# full diminished chord contains the following notes,

G#, B, D, F. Note that the note F is not a part of the A major scale, so a G# full diminished chord is not played as the seventh chord of the a major chord progression. That would be G#m7b5 (half diminished).

I hope that you understood the theory behind diminished chords.

answering the second part of your question i.e. name of the chord: root, dim3, dim5. It is somewhat wrong. It should be root, minor 3rd, diminished fifth. This is not a chord. This is what is called a diminished triad. I have explained this above. Adding a bb7 or a b7 would make full diminished and half diminished chords respectively.

One more important thing. In a full diminished chord i.e. 1, b3, b5, bb7, all the notes are equally spaced so there is no distinct root. The root is ambiguous. All four notes can serve as a root. As a result, four different diminished chords (full dim) can be played with exact same shape and position. For example, the notes A, C, Eb and F# make the Adim7, Cdim7, Ebdim7 and F#dim7. dim7 is a notation used for full diminished chords. So, playing Adim7 is the same as playing Cdim7, Ebdim7 and F#dim7.

Notation used for half diminished chords is m7b5 or chord root followed by a superscript o with a strike through. Notation for full dim chord is dim7 or (note)°. For example, A°, B° etc.

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  • A diminished chord has only three pitches, which is why the name "diminished seventh chord" exists. – phoog Jun 4 at 3:21
  • A diminished seventh in key A would be Gb, not F#. Enharmonic, yes, but F# is M6, not d7. – Tim Jun 4 at 7:52
  • @tim you are right. The dim 7 seventh is indeed Gb in the key of A but that's not there concern. While constructing the vii chord of the progression of A three seventh note which is G# is to be considered as the root. From there you have to start counting. For example, the scale of G# is G#, A#, C, C#, D#, F, G, G#. So if G# is the root then Cb would be b3(min3), D would be b5 and F# would be b7 and these notes are found in there key of A and the chord will not sound dissonant. These notes make the half dim chord. In a full dim chord the F# would become an F which is not in scale of A – John1085 Jun 4 at 13:22
  • @tim the dim chord in the key of A is the vii chord and its root is the seventh note which is G#. So you have to count the notes starting from G# and in the scale of G#. You should read the sixth paragraph of my answer which starts with "a full diminished chord". I hope that I've made myself clear. – John1085 Jun 4 at 13:27
  • @phoog what you have mentioned is the diminished triad not the diminished chord. – John1085 Jun 4 at 13:28

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