In Second Species counterpoint, two tones are used against each note of the melody. It is important to note where one places the consonant and dissonant tones.
Accented beats on the counterpoint should create consonant intervals. The exceptions for this are that the perfect unison should only fall on unaccented beats until the last note of the song and the octave should be used sparingly on accented beats.
Dissonant tones can be used on the weak beats provided that it is used as a passing tone.
A rest may be used in place of the first beat for the counterpoint. This is the only place a rest is allowed. In this case, the unaccented beat must have a perfect consonant interval. If the cantus firmus is above, this would refer only to the octave or the prime. If below, it refers to the octave, the prime, and the fifth.
It is often best to start by writing the cadence first.
When writing the cadence in second species counterpoint, you have to think about whether you are writing your counterpoint above or below the Cantus Firmus.
Writing cadences above the Cantus Firmus Edit
If your Cantus firmus ends in 2-1, then your counterpoint will end with (6-7)-8. Often the 5 will proceed the 6.
If your Cantus Firmus ends in 7-8, then your counterpoint will end with (2-5)-P
Writing cadences below the Cantus Firmus Edit
If the cadence of the Cantus Firmus is 7-8 then the counterpoint will end with (5-2)-1.
If the cadence of the Cantus Firmus is 2-1 then the counterpoint will end with (6-7)-8.
NOTE: Numbers in () show that they are half the duration of the others.