Here we go, here we go, here we go..
Cantus firmus in 6 modes.
Why no locrian? I'm guessing because it has a dissonant root chord due to the lowered 5th.
What are modes? ROUGHLY - imagine these are all melodies written in C major, the "ionian" mode is the standard major scale. The "dorian" is like C major, but D is the root, instead of C. In each subsequant mode a different scale degree from C major is the root note. You can tell what the root is because in all cases its the first and last note of the cantus firmus.
Ionian

Dorian

Phrygian

Lydian

Mixolydian

Aeolian

The objective here would be to write a second part either above or below each of the cantus firmus (or just 1 or 2 if thats enough for you). Just write 1 semibreve for each semibreve in the C.F. and always use consonant intervals, that is, unisons, 3rds, 5ths, 6ths and 8ves.
Additionally, if you are particularly game, there are some basic rules to consider..
Unisons, 5ths and 8ves are Perfect intervals
3rds and 6ths are imperfect
- From on PERFECT consonance to another
PERFECT consonance must proceed in contrary or
oblique motion - From PERFECT consonance to IMPERFECT
consonance one may proceed in any of three motions - From IMPERFECT consonance to PERFECT
consonance must proceed in contrary or oblique
motion - From IMPERFECT consonance to IMPERFECT
consonance one may proceed with any of three
motions
^ if too much thinking maybe just ignore that temporarily.
Also - try to use STEP motion (part moves in 2nds), rather than LEAPS (part moves in 3rds or greater) for most of your part, AND where you use a leap, you will often then go back in the opposing direction (as in if you leap up in pitch, you will probably then step (sometimes leap) down in pitch) If you observe the cantus firmus examples you will see this is observed
most of the time.. there are ofcourse exceptions.. think "guide" note "rule"
I will post my attempts in due course, though I will be at a wedding tonight so it will probably be late tomorrow before I do it.
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EVERYONE please tell me if this is too much for you..