figure 1figure 1a
consonant ligatures(bad) consecutive 5ths
figure 2
dissonant ligatures,
suspension & resolution
figure 3
(bad) consecutive unisons

Fourth Species

FUX says that this species is called ligature or syncopation. The rules for fourth species are:

Although it is permissible to use consonant minims without ties occassionally, this should be avoided if possible. In the rare cases where it is not possible to use ligatures and untied minims must be used, ties should be reintroduced at the first opportunity.

An example from Gradus

Note how Fux writes consecutive octaves in bars 9 & 10, despite his earlier warning. He does this in another example, too; it may be significant that in both cases a leap of a 3rd (consonant ligatures) is involved, rather than a step.
Also note that the closing formula requires preparation in the bar preceeding the penultimate one.

How to use the applet

The basic procedure is the same as in the preceeding applets, though the introduction of ties causes some slight changes. When the user clicks in the second half of a measure, a minim is entered there and tied to another minim which is simultaneously entered on the first beat of the next bar. When the user clicks in the first half of a measure, an untied minim is entered on the first beat of that bar; if a tied minim was previously there, the tie is removed from the minim in the preceeding bar.

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