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Why is it in counterpoint that the fourth degree of natural mode "F" lowered to "Bb" instead of remaining as "B"? Why isn't the fourth degree lowered for other modes? Shouldn't the "B" remain natural if composing within the natural mode of "F"? Why does go outside the natural mode?

Transferred from: Few Questions on Counterpoint in the Tradition of Johan Fux

Not a duplicate, but similar to: Accidentals in First Species Counterpoint

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The fourth degree was changed to "Bb" because if it were left as "B" natural, the resulting interval would be a tritone. This is not the case for any of the other modes - in all other modes, the relationship between the root and the fourth degree is a perfect-fourth. Therefore, such alteration is necessary for writing consistency across the modes. In this way, if the "Bb" were left as a natural, Fux technically would have been going "outside" the mode.

Transferred from: Few Questions on Counterpoint in the Tradition of Johan Fux

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Note that the tritone inverts to itself. Therefore since B is a tritone above F, likewise F is a tritone above B. In other words, the "B mode" (Locrian) also has a tritone above the root. It's just not the fourth degree! (That being E, a perfect fourth). Nevertheless, if we want some mode over B which has a perfect fifth, like B major, we have to fix that by sharpening the F, otherwise we will be outside of the mode. –  Kaz May 31 '13 at 3:46
    
@Kaz - please see my comment in response to the answer you posted earlier. Counterpoint was not written in the locrian mode for the reason of the tritone. If you read the original question, you would have understood that the question from the original thread addressed the "F" mode specifically - not the "B" mode, which, as I pointed out earlier, was not used. –  jjmusicnotes May 31 '13 at 4:44

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