In fourth species the added voice is in syncopated whole notes.

Each whole note is displaced by a half note; we write it as tied half notes.

In consonant fourth species al parts of the syncopated whole are consonant (consonant ties).

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Dissonant fourth species is the only situation in which downbeats are dissonant.

The dissonant note has to be tied from the previous (same) note (the consonant preparation of the dissonant).

The dissonant resolves with a downward step towards a(n) (imperfect) consonant.

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Use 7-6 and 4-3 and their compounds as dissonant suspensions above the CF.

Use 2-3 and its compound as dissonant suspensions below the CF.

 

Hard Rules

  1. Dissonance on the downbeat is allowed in syncopated whole notes when:
    the first half of the note is consonant (perfect or imperfect)
    the dissonance is followed by a descending step to an imperfect consonance on the second half-note beat
    So: don't tie from a dissonant, or skip from a dissonant
  2. If you move back to second species temporarily, the last suspension must be completed with an imperfect consonance on the weak beat
  3. Don't fall in the trap of "fake suspensions":

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(Don't write too many prepared dissonances, when writing syncopated whole notes)

 

An example by Diruta

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