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In my old theory book, I read that while ii and ii6 harmonize scale degrees 2 and 4, they rarely harmonize scale degree 6 in the soprano. (I'm assuming they mean for common practice music, SATB). What would be the reason behind this? Are there other things to know about that might be related? For example, I've seen advice about the "II" chord (any quality) in minor keys, due to it being "diminished", where the melodic minor had not yet been introduced, so these other possibilities were not mentioned.

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    I like to remind people that the 'rules' that theory dictates are not actually rules but guidelines, which are specific to a genre or an era within a genre. One example is parallel fifths; not allowed in Classical but exist constantly in most other genres. So I might suggest that this is less a rule about scale degree 6 not being well harmonized by ii than a rule about how to make compositional choices that would align with the standards of the Romantic era. After all, if you are trying to compose a piece of Baroque music, you wouldn't use the same approach (rules) as a Romantic piece. – Basstickler Mar 2 '16 at 14:51
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    I tend to think of theory as an explanation for why things sound good than a set of rules, which seems to align with the great composers, who all broke the rules of their time, essentially creating new possibilities, which become rules, as a result. – Basstickler Mar 2 '16 at 14:53
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This seems to me to be a strange statement.

For one thing, unless the chord is tonicised as ii, the analysis of the chord as ii is not the only possible analysis. (One way to tonicise it as ii is to precede it by V of ii.) It could also be said to be functionally IV, with 2 being either a non-harmony note or a substitute note for 1.

It seems to me that the question means "When the tune has 6, why is ii rarely used to harmonise it?" -- have I interpreted it correctly? If so, then, for another thing, I'm not sure that it is rare. (I'm analysing the chord as ii anyway, for the sake of argument.)

Mozart is certainly common practice music, so I looked for examples of ii harmonising 6 in his string quartets in major keys. Here are some:

  • Quartet 14 in G K387, mvt.1, b.19, 37 (3rd beat)
  • Quartet 17 in Bb K458, mvt.1, b.32
  • Quartet 18 in A K464, mvt.1, b.86. Also b.59, 63 unless the main pitch in the 1st violin is considered to be A rather than C#.
  • Quartet 19 in C K465, mvt.1, b.40, 42, 69
  • Quartet 20 in D K499, mvt.1, b.86
  • Quartet 22 in Bb K589, mvt.1, b.41
  • Quartet 23 in F K590, mvt.1, b.5 (after the appoggiatura resolves)
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Without a reference to the book, or a longer quote, any answer is just a guess. "Old" theory books are often very prescriptive, when compared with what composers actually wrote.

That said, in common practice harmony ii and ii6 are often the start of the perfect cadence formula ii-V-I. If you don't want to make a cadence, IV and vi are perfectly good chords to harmonize scale degree 6.

  • Thanks. I've discussed this with two teachers, and all three of us came up with notes that would work, and without that great an effort. The book is the previous edition of Sarnecki, book 1 (2002) so it's not that old, but maybe it's quoting older sources. When I was googling it, someone out there had the same rule in ABRSM, and again without explanation. What interests me in theory is the understanding part. Then you also need less rules, if any. Btw, the same book later lists 6 as one of the degrees that ii and ii6 harmonize. – keys 'n strings Sep 5 '15 at 1:07
  • In my answer "old" meant 50 years old, not 2002. But there are lots of more recent books are basically the next link in a game of Chinese Whispers. I can't find much on the web about Mark Sarnecki, except that he wrote plenty of textbooks. A textbook for a first university course on harmony might be better for "understanding," once you know the basic terminology - but the best tools for "understanding" are two ears and one brain, not textbooks. – user19146 Sep 5 '15 at 13:50
  • Well, when I asked the question, it was in case anyone might have actually run into an explanation of the thinking behind this (assuming there was any). I like your metaphor of Chinese whispers. The theory I'm doing with my teacher veers more to Romantic and later, and it's not a 100% conventional path - here it is indeed ear etc. To me the rule doesn't make sense because I could harmonize the 6 using ii. A musician / teacher lulled herself to sleep making up a dozen. Up to now, the rule doesn't make sense. – keys 'n strings Sep 6 '15 at 14:03
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    I now have a tentative answer from someone who teaches theory. "Regarding the ii chord harmonizing the 6th scale degree, the approach I-ii with a soprano ^5-^6 would result in parallel fifths, and ii is often approached by I." So it would not actually have to do with ii going to V, but I going to ii. – keys 'n strings Sep 7 '15 at 19:54
  • That is a general "problem situation" with any chord progression where the melody moves the same number of scale-steps and in parallel with the roots of the chords. The inner two parts need to move in contrary motion to avoid consecutives, and that can be difficult to achieve unless the voices are widely spaced. – user19146 Sep 7 '15 at 20:44
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Just to add AlephZero's answer the ii6 chord is the standard for the perfect cadence because it makes the big sense if you look at the bass line.

If you have the Cadential 6/4 progression you could have ii6-I6/4-V-I. That tonic chord in the second inversion is just a decoration of the dominant chord. If you look at the very important bass line you now have for C Major for instance have. F-G-G-C.

This is a standard chord progression that everyone should know for their harmony exercises.

In my old theory book, I read that while ii and ii6 harmonise scale degrees 2 and 4, they rarely harmonise scale degree 6 in the soprano

The supertonic is an excellent choice of chord after the Sub-Mediant chord.

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