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From this answer given by @jjmusicnotes:

2.) The term "diatonic" refers to what naturally occurs in the scale. In major keys, each degree of the scale corresponds to a different mode - even the root pitch. The exercises are in modes that are for the most part diatonic. The exceptions include avoidance of dissonance and clausula vera which you can see in the exercise that it is the less common motion-structure - from a third to a unison.

Can you explain the exceptions and give examples.

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This should probably be a comment on his answer –  American Luke May 27 '13 at 17:53
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closed as not a real question by Monica Cellio, Jason W, ecline6, nonpop, American Luke May 31 '13 at 23:01

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1 Answer

Clausula vera is the most common cadence in two-voiced, sixteenth-century choral writing. The voices either expand to an octave from a sixth or contract to a unison from a third. This type of cadence is heavily prescribed throughout The Study of Counterpoint.

It is done by step, not skip, and as far as I know is not exclusive to raised sevenths in the given mode, though most examples I have seen include altered pitches.

That said, I would like to redact a portion of my original statement as I had a mis-thought. I would like to instead reference and substitute the term musica ficta, which is actually more in-line and appropriate in answer to your original question.

You can read more about Musica Ficta here.

Sometimes when writing in a mode, say, A Aeolian for example, it is necessary to alter pitches in the original mode to either avoid dissonances or to bring about a strong sense of conclusion at the end of a piece or section. Alternatively, a composer might alter specific tones in order to prepare for a modulation, tonicization, or deceptive cadence. Moreover, composers of that time also sometimes considered alteration of some notes as non-functional: only valuable for a shift in musical "color".

Essentially, in this time period, whenever you see an accidental, you can bet musica ficta is happening.

Hope that helps clarify things, and I apologize for the mis-statement in my original answer. I will edit accordingly.

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