Text: Of the above, the octave and the perfect fifth are classified as perfect consonances. The thirds and sixths, both major and minor, are imperfect consonances. The seconds and sevenths, both major and minor, are dissonances. The perfect fifth, the augmented fourth and the diminished fifth are ambiguous in character. (Theoretically, the perfect fourth is per se consonant; the augmented fourth and diminished fifth are both dissonant.) In practice they are treated as dissonances when the bass forms the lower note. But when they occur between two of the upper constituent notes of a chord, they rank as consonances, inasmuch as they require no prescribed method of resolution, as do the authentic dissonances. The augmented fourth and diminished fifth are included in the above list because they may be diatonic intervals formed between the fourth of the scale and the leading note or vice versa. In addition to these, the perfect fifth, the major second, and the major sixth may be augmented by raising the upper or lowering the lower note chromatically. Similarly the minor third, perfect fourth, minor sixth and minor seventh may be diminished by lowering the upper note or raising the lower note chromatically. These are the only augmentations and diminuitions in practice. Others exist in theory; but only the paradoxically-minded need spend any time in considering, for example, just what kind of a second is formed by the interval B# - Cflat. The unison is the most perfect of all consonances, but there seems no good reason to follow the usual practice of including it in the list of intervals. Its very name implies that there is no interval between the two sounds which form it. Any intervals listed above becomes what is known as a compound interval if the upper note is raised or lower note lowered an octave, or both. Any chord which contains no dissonant interval is a concord, any chord which does contain a dissonant interval is a discord. The ambiguous nature of certain intervals will explain how it is, for example, that a diminished triad in root position is a discord, its first inversion a concord, and its second inversion once more a discord. It all depends on the position in the chord of the augmented fourth or diminished fifth.

See Also: VIBRATION RATIO

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