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.
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.
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.
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.
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' chords, if 'diatonic' means using only the notes from the tonic scale. But they fit pretty well BETWEEN diatonic chords, without confusing the key centre.