Intervals can be classified as consonant or dissonant according to the complexity of the mathematical relation between the pitches of the notes.

Although this concept has changed throughout musical history and not all theorists agree, we can offer the following classification:

Consonant	Dissonance
Unison	Second
Major and minor third	Perfect fourth (considered a dissonance in harmony and counterpoint)
Perfect fifth	Seventh
Major and minor sixth	Augmented fourth
Perfect octave	Diminished fifth

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