imperfect consonance, as in the first species, but the fifth, or octave,
may be used occasionally.

In this and succeeding lessons, all notes in the measure not belonging to
the harmony implied on the first beat, must be treated as dissonances,
e.g., those belonging to the implied harmony may be left by a skip (_a_) or
stepwise progression (_b_) unless dissonant with the cantus firmus; then
avoid their use; if foreign to it, whether consonant (_c_) with the C.F.
or not (_d_), they must be treated as embellishments or passing-tones.
[Fig. 21.]

[Illustration: Fig. 21.]

The embellishment may be used as follows: when above the principal tone, it
may be a semitone (_a_) or a whole tone (_b_) distant from it; and when
below, a semitone (_c_). [Fig. 22.]

[Illustration: Fig. 22.]

When the counterpoint is below the cantus firmus, the fifth of the chord
needs special treatment. It is permitted on the weak beat when the lower is
treated as an harmonic passing-tone.

An harmonic passing-tone is the second of three tones belonging to the same
chord. [Fig. 23_a_.] While the third tone should be a member of the chord
containing the fifth as an harmonic passing-tone, the chord above it may
change as in Fig. 23_b_.

The fifth is permitted on the strong beat when it is only an implied fifth
(six-four chord); that is, the third and fifth appear on the strong beat,