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
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
If you are still referring to bar 5 of Fig. 14 p. 36. then I guess that the B of the cpt is flattened due to the F of the previous bar in the c.f. Although this is not mentioned in this book, the devil in musica is to be avoided even when it is singed by different voices. This is also the case when dealing with more voices if the tritone is formed in the outer voices.
