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IX. §§ 98, 99.] INVERSION OF INTERVALS. 191
obviously mutual, so that each may be described as the inversion of the other. Accordingly the inversion of the Major Sixth is the Minor Third.
The following table1 shows the three pairs of consonant intervals narrower than an Octave which stand to each other in the mutual relation of inversions.
98. A combination of musical sounds of different pitch is called a 'chord.' Hitherto we have considered only chords of two notes, or ' binary' chords. We now go on to chords of three notes, or as they are usually called 'triads.' A triad contains three intervals, one between its extreme notes, and one between the middle note and each of the other two. In order that the chord may be free from dissonance, those intervals must all three be concords.
99. We may, then, search for consonant triads
in the following manner.. Having selected the lowest
of the three notes at pleasure, choose two others each
of which forms with the bottom note a consonant
interval. Next, examine whether the interval formed
by the last chosen notes with each other is also a
1 The last two results of this table will be easily verified by the student.