A pitch is 'dissonant' if it forms an unstable interval with other pitches. Stable or 'consonant' intervals are the octave, fifth, thirds and sixths. Unstable or dissonant intervals are the others: seconds, sevenths, and particularly the augmented 4th and diminished 5th. Any augmented or diminished interval will also be considered dissonant in part-writing, though some of these (e.g. the diminished fourth) actually sound, on a piano, the same pitches as consonant intervals. The octave and the fifth are known as "perfect consonances" and the thirds and sixths are "imperfect consonances."
The perfect fourth is a special case: it is treated as a dissonance if one of its pitches is the bass (lowest tone in the sonority).
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