The first quarter note exists on the downbeat of the measure. Because of this, it must be consonant with the cantus firmus. The second and fourth notes may be dissonant with the cantus firmus if we both approach and leave them by stepwise motion; and the first, third and fifth (1st of the next measure) notes in the sequence are consonant with the cantus firmus. The third beat may be dissonant with the cantus firmus by way of diminution. In this case the second and fourth notes are always consonant with the cantus firmus.
Works of third species counterpoint will always resolve in a specific way. If the counterpoint resolves to an octave, the penultimate measure will contain a 5th followed by a major 6th in relation to the cantus firmus in the third and fourth beats (except when the diatonic 5th is a tritone). If the counterpoint resolves to a unison, the penultimate measure will contain a 5th, followed by a 4th and then a minor 3rd in relation to the cantus firmus in the second, third and fourth beats (except when the diatonic 5th is a tritone). The final pitch of the counterpoint must match the rhythm of the cantus firmus (in our case, it will be a whole note).
There is one other exception to the normal rules of counterpoint that can occur in third species. This exception is due to a type of musical phrase called a cambiata. Cambiata is an Italian word meaning changed note. In other types of musical composition, the specific parameters of a cambiata may vary. In 3rd species counterpoint, this is specific type of melodic phrase containing a step, followed by a skip of a 3rd in the same direction and finally a step in the opposite direction (which fills in the skip). In some cases, this type of phrase fits within the normal rules of counterpoint.
Based on the rules and tips presented in this lesson, you should be prepared to compose your own third species counterpoint exercises. For more guidance, please refer to the end of the accompanying video where I compose an example of first species counterpoint both above and below a cantus firmus. Continued practice will grant you valuable insight into the nature of melodic motion and the way multiple melodies react harmonically.
This Learning Music With Ray video discusses the topic of third species counterpoint. In this video I discuss the rules that govern composing a work of third species counterpoint. I also provide some helpful tips that will make your experience composing third species counterpoint easier. Finally, I compose a line of third species counter point both above and below a cantus firmus in order to provide a live demonstration of the principles discussed in the video.