In third-species counterpoint, the counterpoint line moves in quarter notes against a cantus firmus in whole notes. This 4:1 rhythmic ratio creates a still greater differentiation between beats than in second species: strong beats (downbeats), moderately strong beats (the third quarter note of each bar), and weak beats (the second and fourth quarter notes of each bar). Third species also introduces the neighbor tone dissonance, and two related figures in which dissonances can participate in leaps.
As in first and species, the counterpoint line should be singable, have a good shape, with a single climax that does not coincide with the climax of the cantus firmus, and primarily stepwise motion (with some small leaps and an occasional large leap for variety). Like second species, a third-species counterpoint should be even more dominated by stepwise motion than in first species, because there are less sticky situations that would require a leap. If the counterpoint must leap, prefer to do so within the bar rather than across the barline. Also like second species, there should usually be one or two secondary climaxes—notes lower than the overall climax that serve as "local" climaxes for portions of the line.
Begin a third-species counterpoint above the cantus firmus with do or sol. Begin a third-species counterpoint below the cantus firmus with do. Unisons are permitted for the first and last dyads of the exercise.
A third-species line can begin with four quarter notes in the first bar, or a quarter rest followed by three quarter notes. Regardless of rhythm, the first pitch in the counterpoint should follow the intervallic rules above.
The final pitch of the counterpoint must always be do, and must be a whole note.
The penultimate note of the counterpoint (the last quarter note of the penultimate bar) should be ti if the cantus is re, and re if the cantus is ti.
Principles for strong beats (downbeats) are generally the same as in second species.
Strong beats should always be consonant, and should not be unisons. Prefer imperfect consonances (thirds and sixths) to perfect consonances (fifths and octaves).
Motion across bar lines (from beat 4 to downbeat) follows the same rules as first species counterpoint.
Progressions from downbeat to downbeat follow principles of second-species counterpoint, with one exception (see below). The following are some examples, but not an exhaustive list:
If a downbeat contains a perfect fifth, neither the third or the fourth beat of the previous bar can be a fifth. If a downbeat contains an octave, neither the second, third, or fourth beat of the previous bar can be an octave.
Hidden or direct fifths/octaves between successive downbeats are allowed.
Beats 2–4 should exhibit a mixture of consonant and dissonant intervals. Among consonances, unisons are permitted on weak beats when necessary to make good counterpoint between the lines. Any dissonance must follow the pattern of the dissonant passing tone or the dissonant neighbor tone, explained below. Also explained below are a number of standard patterns for consonant weak beats.
Generally, dissonances in third species can occur on beat 2, 3, or 4, and should be preceded and followed by stepwise motion (with the exception of the double neighbor and the nota cambiata, explained below). If all dissonant notes in the counterpoint follow one of the following models, they should have a pleasing effect. If not, they may sound harsh or unresolved, or will be difficult to sing.
The dissonant passing tone fills in the space of a melodic third via stepwise motion. The notes before and after the passing tone must be consonant with the cantus.
Note that it is possible to have two dissonant passing tones in a row (P4–d5 or d5–P4). As long as these dissonances do not fall on downbeats and the counterpoint moves in stepwise motion in a single direction, it is permissible to have two dissonant passing tones in a row.
The dissonant neighbor tone ornaments a consonant tone by stepping away and stepping back to the original consonance (6–7–6 over the cantus, for example). It is melodically identical to the consonant neighbor tone of second species, with the difference being the harmonic dissonance. It is best employed on beats 2 and 4.
The double neighbor occurs when beats 1 and 4 in the counterpoint are the same tone, and beats 2 and 3 include the notes a step higher and a step lower than the original tone. For example, C–D–B–C or C–B–D–C. Both beats 2 and 3 are dissonant, but since both are embellishing the original tone by step, the leap between them sounds fine and is easy to sing. When using a double neighbor, the direction between beats 3 and 4 should be the same as between beat 4 and the following downbeat. That motion across the barline should also be stepwise.
The nota cambiata (changing tone) is a five-note figure that outlines a step progression from downbeat to downbeat. It follows one of two patterns:
The first pattern will result in a step down from downbeat to downbeat, and the second pattern will result in a step up from downbeat to downbeat. For a nota cambiata to be effective, the first, third, and fifth notes must be consonant with the cantus. The second note will be dissonant and will leap to the third tone, but that is fine if the entire figure is followed precisely.
The counterpoint can move in and out of consonant tones freely by step, as well as by leap from another consonance, with the following considerations:
There is no demonstration video for third species.