Need help with Fux

I've just started to work on Joseph Fux' The Study of Counterpoint (unfortunately without a teacher. There's no one around here, but that'll hopefully change when I move this autumn!). I enjoy it a lot, but the a lot of the music theory in it is alien to me.

I may make this a returning thread, where I ask questions as I encounter them, but for now, there's only two things I'm wondering about:

1. Why both a g and c clef? (obviously, the c clef is the valid one)
2. I don't understand the error made on page 31 (Alfred Mann edition). At the risk of breaking copyright laws, I'll include a small quote:

"...the cantus firmus is in D (la, sol, re), as the beginning and conclusion show, and you started with G (sol, re, ut), you have obviously forced the the beginning out of the mode".

What exactly does mode mean here? I know THE modes, but it must have some other meaning in this example?

1. Why both? If I remember correctly, it's because they were originally written (say) in soprano clef, which has been updated to (in this case) treble clef.
2. I'd have to read it in context, but the pupil (Josephus?) has obviously used notes not from the correct mode, in which the cantus firmus is written, and the counterpoint should be in.

Here is a transcription of the musical example (for simplicity, I notates it as quarter note). The first G in the Josephus voice was the wrong note, the the D the corrected note. I've looked over it, and I still don't "get" why the perfect fifth interval don't fit here.

If it's Dorian, which it looks like, how could it start on a G and a D? If you follow through the rest of the cantus, it doesn't make sense to start on a "G chord". Examples like that pretty much always start and end in the same 'key'.

Of course! Since Josephus' counterpoint voice is the lowest one this time, it is the one who establishes the mode, right? If the voiced switched places, so that the D would be the deeper note, and G the higher, there wouldn't be a problem, right?

In Dorian D and D or D and A would work. Not D and G or G and D.

Now I think I get it: A D is the fifth of a G, which would imply that G would be the mode (or the tonic of the chord if we were talking about harmony). On the other hand, A is the fifth of D, making D the mode. Is this the reason?

Originally Posted by Halvor Hosar

Need help with Fux

Sounds like a personal problem.

Apparently, Fux is very popular in school choires because his name often is mispronounced.

Okay, I've gotten to part two of the book (three voice counterpoint), but I'm still a bit uncertain about things. On page 35, it is explained why mi (F or B) against fa (G or C) is not allowed to use. That's fair enough, because these are small seconds that weren't percieved as good sounding. However, it is also explained how you can use sharps or flats to avoid this. But wouldn't this just move the problem? If you look at the example I included, it's true, mi and fa is not used against each other, but you still have a minor second in there, this time between A and Bb. Is there any specific reason why it is these exact minor seconds that aren't allowed, while others are fine?

Secondly, I've just started out on the second part, where you use the first species of counterpoint in three voices It is explained how the standard triad is considered the ideal, and why other combinations may have to be used (perfect intervals != direct motion). The book suggests that 1 3 6, 1 3 8 and 1 6 8 are accepted substitutes. However, I would presume that when writing in three parts, you have to look at the relationship between each of the voices. In this case then, 1 3 6 would contain a third and a fourth, a dissonant interval not to be used. How come it is accepted here? And why wouldn't 1 5 8, containing a fifth and a fourth be accepted then?