Reference
Consonance and Dissonance
Intervals can be classified as consonant or dissonant according to the complexity of the mathematical relation between the notes pitches.
Although this concept has changed during musical history and even today not all theoreticians agree, we can offer the following classification:
Consonant Dissonance Unison Major and minor third
Perfect fourth
(considered a dissonance in
harmony and counterpoint)Perfect fifth
Major and minor sixth
Perfect octaves
Seconds Augmented fourth
Diminished fifth
