Audio: contrapuntal consonance (0:06)
contrapuntal consonance sings first species counterpoint in the key of C major. The contrapuntal consonance figure shows that all the intervals in the score are consonant. The contrapuntal consonance table lists the intervals and their status in counterpoint and classifies them into perfect consonances, imperfect consonances and dissonances.
Consonance focusses on the vertical dimension of counterpoint, the interval. Every interval is classed as consonant, pleasing to the ear, or dissonant, not pleasing to the ear. Consonant intervals are further classed as perfect and imperfect. An imperfect consonance is not faulty in any way, it is a major or minor interval as opposed to a perfect interval.
The consonant intervals of third, fifth and sixth are allowed in all species of counterpoint. One dissonant interval, the tritone, is banned in all species of counterpoint. The status of the remaining intervals varies, meaning they are allowed in some species, but not others.
Treating the perfect fourth as a dissonance is a big surprise. The inverse of the perfect fourth, the prefect fifth, is treated as a consonance, so one would expect the fourth also to be treated as a consonance. There is no logical rationale for treating the fourth as a dissonance, it is simply due to usage and custom.
In first species counterpoint the interval between the two parts is always consonant. There are no dissonant intervals in first species counterpoint. Unison, a consonant interval, is only allowed at the beginning and end of the piece and not during it. The jury is still out on the octave, a consonant interval, some allow it, some do not. The remaining consonances of third, fifth and sixth make up the bulk of first species counterpoint. In practice, thirds and sixths predominate, not just in first species but in all species of counterpoint.