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I am learning about repetition in classical composing using a basic idea that prolongs tonic (is that what they always do?). Anyways, I use a passing chord between I and vi, and fully diminished voiced with no repeat of the root an octave up provides a nice pull to vi, for which I did use the root an octave up. My question is, does fully diminished (or is it usually referred to as dim 7th?) vii fit into diatonic harmony? Also, what scale should I use if I'm following the strict guidelines of a composer writing chord tone melodies for exercises (I don't know if that's the correct term, chord tones on the strong beats is what I mean). I realize this is like 5 questions but most of them are feeling out the same concept I think (diatonicism vs chromaticism). Thank you.

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    by what scale I meant for diminished or fully diminished, and i simply meant I can't draw outside the lines right now if I want to learn the concepts – Frank Badertscher Mar 22 '17 at 21:55
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    It might be useful to clarify what you mean by "fit into." The diatonic scale is a heptatonic scale, while diminished chords are associated with an octatonic scale. So it's no surprise that there is no diminished mode in a diatonic scale. If this is what you mean by "fit into," then the answer would be "no, diminished chords don't fit in." Is your question the whether vii o7 chord can be used in a song with deviating from a previously-established diatonic tonal center? Are you thinking in terms of a major tonal center, minor tonal center, or both? – jdjazz Jun 12 '17 at 22:26
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A fully diminished 7th chord is home on the 7th scale degree of harmonic minor. Technically this is outside the diationic scale, but is well within the common practice period's style as are other chromatic concepts such as augmented 6th chords and Neapolitan chords. In C harmonic minor which would consist of the notes C, D, E♭, F, G, A♭, and B, this would be Bdim7 with the notes B, D, F, A♭. Due to the symmetrical nature of the fully diminished 7th chord can also be looked at as Ddim7 and ii° respectively which also wants to take you back to i.

Don't get too hung up on the idea you have to strictly be using harmonic minor to use this as the chord itself is chromatic in nature and in during the common practice period especially in minor keys you will shift through the natural, harmonic, and melodic minors and it would not be too far fetched to see this chord in a major key.

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    If you add just a small bit about the function of this sort of ii o7 chord (e.g., to lead into a minor i chord), I'll upvote. I think this is an extremely clear and appropriate answer given the level of the question. – jdjazz Jun 12 '17 at 22:30
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    @jdjazz updated my answer to include the information from your comment. – Dom Jun 13 '17 at 15:40
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C, C#dim7, Dm7, G7, C. Or C/E, Ebdim7, Dm7, Db7, C. Both are very common sequences, neither depart from the key of C major, although they use chromatic notes.

No, they aren't 'diatonic' harmony, if 'diatonic' means using only the notes from the tonic scale. Is there some requirement to be diatonic?

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    Justify the downvote, please? – Laurence Payne May 23 '17 at 15:07
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    I can't, so I voted it back up. But your answer is essentially a jazz answer to a question of classical harmony. "Diatonic" indeed means using only the notes from the tonic scale, and also means "not chromatic." The relationship between chords and structure in jazz is quite different than it is in truly classical (say, pre-Debussy) harmony; what is simply a member of a chord progression in jazz harmony will often imply an upcoming modulation in classical harmony. For example, a C#dim7 in the key of C would imply an upcoming resolution to a B minor tonic. – BobRodes Jun 13 '17 at 18:23
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    Not that different! Bach, Beethoven (and all the rest) used C#dim7 as a simple passing chord just as often as they used it as a gateway to a remote key. – Laurence Payne Jan 13 at 14:11
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    I can't think of any examples of this (although there may be plenty, perhaps you can give me an example), and I can think of many examples of dim7 chords used as a modulating device. Also, my general point I believe is true, that jazz uses 7th chords where someone like Mozart would use a simple triad -- while jazz players say that simple triads are fine, in practice they don't use them that often, if at all. For example, if a song in a fake book has an F chord, that means an F major 7 chord, not a simple F triad (am I wrong?). Not what you would see in Bach's time. – BobRodes Jan 13 at 23:27
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    By the way, it wasn't I who downvoted your answer. :) – BobRodes Jan 13 at 23:29
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Common Practice Period music describes the minor mode as having two mutable tones, 6 and 7. In a minor key, using either form of either note is still considered as diatonic. For that matter, many chromatic chords may be used without effecting a key change: secondary dominants, augmented sixths, Neapolitan sixths, various diminished and half-diminished sevenths, and even a change of chords from major to minor or vice versa.

Classical (CPP) melodic analysis generally discusses "non-harmonic" tones rather than extending chordal types (passing tones, neighbor tones, accented or unaccented, escape tones, suspensions, etc.) There are a few tendencies in the use of raised vs unaltered degrees 6 and 7 in a minor key. Often, these tendencies are ignored because a composer thinks that would sound better. A few of these are given below.

Ascending scale runs with tonic harmony uses raised 6th and 7th steps. Descending scale runs with tonic harmony uses lowered 6th and 7th steps. Ascending or descending runs with dominant harmony uses raised 6 and 7th steps. Ascending or descending runs with subdominant harmony uses lowered 6th and 7th steps. Arpeggios of a dominant ninth chord use the lowered 6th and raised 7th step. The upper neighbor of the fifth step is the lowered sixth step (I didn't find any exceptions here.) The augmented III chord is used rarely. The most common exception seems to be scale runs with a lowered 6th and raised 7th step. Sometimes this is used to give an exotic or oriental flavor the music.

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    From the melodic minor standpoint, say, using Cm, would the 6th as A be called augmented, or would using the 6th as Ab be called diminished? As both are actually 6ths in their own right within the key/scale, and both represent different 6ths. So which is taken as the datum point, and why? This is asked because you use the terms 'raised' and 'lowered'. Yes, they are, in a way, but surely one must be the start point, in which case, the other, only, is raised or lowered. It can't be both, logically! – Tim Mar 23 '17 at 10:45
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    @Tim, given that ttw's answer contains "The upper neighbor of the fifth step is the lowered sixth step", the start point is that Ab is lowered and A is raised in C minor. Neither are called augmented or diminished. Now using the A as A# would be called augmented and using the A as Abb would be called diminished. – Dekkadeci Mar 23 '17 at 12:45
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    I tend to use the terms raised and lowered rather than raised and unaltered. Probably because I think (in C) of the A and B being lowered from the Major and raised from the Minor modes. Sorry about the mixed terminology. There are two forms; both are diatonic; perhaps I should stick with the medieval words Hard and Soft. – ttw Mar 23 '17 at 14:00
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If you are up for using a system of thought different from the scaler Roman numeral system, the functional system offers a nice interpretation of the fully diminished 7th chord.

In functional thinking, the B-D-F-Ab chord in c-minor is seen as a dominant chord (G-B-D) with a 7th and b9 added creating a G-B-D-F-Ab chord with the G omitted. The symbol for such a chord is not notatable on computers, it is a Db9, (D for dominant, not for pitch) and the "D" has a slash through it to show the root has been omitted.

In short, it shows the chord as being diatonic, although because it is a major dominant in a minor-key setting, it is technically not fully organic to the minor mode.

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