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Tchaikovsky in his "Guide to Harmony" states the following about the dominant ninth chord (G-B-D-F-A) (translating from the original language):

"Being strongly dissonant, the dominant ninth chord requires a preparation, i.e. the ninth interval (A) should be present in the previous chord in the same voice. Triads on 2, 4, 6th step can be considered as a preparation".

Now, from the previous reading the dissonant chords are "prepared" by supporting consonant chords which have the same tones in the same voices.

Question: Is there any particular reason that we need to prepare specifically the NINTH (A), and not e.g. third, seventh?...

P.S. Yes, I know that the tonic triad can be used before this chord and it does not have common tones.

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    I guess Tchaikovsky preceded the Blues, where none of this is deemed relevant.
    – Tim
    Oct 3, 2019 at 9:09
  • @Tim I understand that those laws are not absolute and may be considered irrelevant by other styles. But still I want to understand them better, not merely learn them by heart
    – NickQuant
    Oct 3, 2019 at 9:14

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"Being strongly dissonant" is the clue. Tchaikovsky considers the 9th a very 'outside' note. As such, it requires preparation.

In the musical style of Tchaikovsky's day, he's got a point. Later styles relaxed their definitions of dissonance, and tolerate (even revel in!) much more angular voice leading. We now happily jump to notes that would, in earlier styles only have been 'allowed' as a suspension, and may even neglect to resolve them. But if you're writing in Tchaikovsky's style, take note of Tchaikovsky's rules.

In the Middle Ages, harmony mainly consisted of doubling a melody at the 5th. Then we got a lot of mileage our of Common Practice harmony, with its emphasis on creating full sounds from smooth but independent voice-leading. Now, in some styles, we barely hear a major 7th or ♭9 as dissonant at all.

What do you feel about the ♯11? If you're a jazzer, perhaps you now consider it commonplace - "I REALLY want there to be a scale that goes with this chord, and the natural 4th doesn't fit, so we'd better sharpen it!" (In older, more melodic days, we'd just have treated the 4th as an 'avoid' note.) Other styles have yet to embrace it. Even in 2019 many musical styles, each with their own 'rules' (or better say 'characteristics') exist side-by-side.

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  • And when do a "mild" dissonance become a "strong" dissonance? Is it about the number of dissonances in the chord OR the presence major, minor seconds and sevenths?
    – NickQuant
    Oct 3, 2019 at 10:12
  • it really depends hugely on taste and context. As others have said, our tolerance of dissonance is far greater now than it was in yeas gone by.
    – danmcb
    Oct 3, 2019 at 10:48
  • @dmb I was asking about the common practice music
    – NickQuant
    Oct 3, 2019 at 13:39
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    But there IS something called 'common practice', @dmb. Look it up.
    – Laurence Payne
    Oct 3, 2019 at 18:06
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    @dmb No, really, 'Common Practice' is a specific and accepted term. "The Common Practice Period was a period of classical music in Western Europe, which spanned 3 centuries from approximately 1600 to 1910" en.wikipedia.org/wiki/Common_practice_period
    – Laurence Payne
    Oct 7, 2019 at 19:21

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