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If I use diminished chords such as ii diminished chord from the minor scale then do I according to the rules for proper part writing use the first inversion of ii diminished. What is the reasoning behind that? Is it because there is no diminished fifth between the bass and any of the upper voices? But there is a diminished fifth between the the fifth and the root. How come the diminished fifth between the fifth and the root is ignored?

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Now that I have answered your question, on re-reading it, I realize I only answered the question for ii-dim, not for other diminished triads. vii-dim serves an entirely different function, though the reasoning behind first-inversion triads is similar. –  Andrew Jun 25 '13 at 5:48

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In common-practice part-writing, the notes of a diminished fifth are tendency tones. That means that the notes tend to go somewhere specific in the next chord. A rule of part-writing is not to double tendency tones. The reasoning is that doing so means that the doubled notes would have to resolve the same way, thereby creating parallel octaves (or unisons, rarely).

Also, in most chords, we double either the chord root or the bass note. Since the chord root of ii-diminished is a tendency tone, we must double the bass, which cannot be a tendency tone. The only non-tendency-tone in the ii-diminished triad is its third, so the chord will be in first inversion with a doubled bass.

Further, the most likely destination of this chord is the dominant, then likely the tonic for (perhaps) an authentic cadence. The third of ii-diminished, which is scale degree 4, can go to the root of V naturally in this circumstance. The motion from scale degree 2 to 3 could not fulfill this tendency (and, in fact, we do not immediately resolve it because scale degree 3 is not in the V chord). Putting scale degree 6 in the bass would mean that the ii-diminished triad is in second inversion, which is unstylistic.

Keep in mind that you may more often use ii-half-diminished-7 as your pre-dominant chord in minor. Of course, iv also works.

I would be happy to clarify if needed. Two seconds of Googling did not yield a satisfactory definition of tendency tones, else I would link to one.

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To expand on Andrew's answer: aurally a fully-diminished chord may function as a secondary leading tone chord. Doubling the root would therefore be doubling the leading tone, which is a big fat no-no. It is always best to put a dim7 in first inversion and double the third of the chord since that tone is also the 5th of the V chord; which sets up a nice suspension for a V6/4 before a cadence. To Andrew: Bach heavily employs iim7b5 chords as predominants in both minor and major, so both are equally acceptable. Tangentially, a iim7b5 6/4 is a great way to modulate by chromatic mediant. –  jjmusicnotes Jun 25 '13 at 21:22
    
@jjmusicnotes A leading tone is, of course, a special case of a tendency tone. I did not address fully-diminished seventh chords, which you probably know have no fewer than eight resolutions and could appear in any inversion. ii-half-dim-7 is, as you mention, sometimes employed as a form of mode mixture. You know, sooner or later, we're going to have an entire undergraduate theory curriculum on here... –  Andrew Jun 26 '13 at 2:41
    
@jjmusicnotes - please elaborate upon the following quote from you: "which sets up a nice suspension for a V6/4 before a cadence." –  Chris Olszewski Jun 26 '13 at 4:49
    
also what do you mean modulate by CHROMATIC mediant –  Chris Olszewski Jun 26 '13 at 4:50
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@Andrew - we're getting pretty close, haha. Since you covered half-dim chords in your answer I thought I'd expand so you wouldn't have to bother. To Chris O. - spell out the chords and work them out! For chromatic mediants - look it up! I do not like to give fish... –  jjmusicnotes Jun 26 '13 at 5:02

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