10: Interval Inversion An Octave or Less

Inversion, at first, can seem rather scary. It isn’t, or won’t be for long, since you’ve already seen half of the concept when you were introduced to the rule of nine. That introduction was inversion in disguise; at that time, to not overload you with information, I simply called it “flipping the interval over.” D to F as D-E-F became F to D as F-G-A-B-C-D, and so on. Unisons became octaves, seconds became sevenths, thirds became sixths, and fourths became fifths. The sum of these numerical parts of the intervals—their sizes—was constantly 9, and this was the so-called “rule of nine.”


There is one more piece to the puzzle of inverting an interval, and that is what to do about the quality when an interval is subjected to inversion. The simplest way to put it is that you pick the “opposite” quality. For four qualities, this is rather intuitive; for the fifth, it isn’t obvious at first.

· Major becomes minor

· Minor becomes major

· Augmented becomes diminished

· Diminished becomes augmented

· Perfect becomes perfect

Here, therefore, is a table of an interval and its inversion:

Start with

Inverts to

Perfect unison

Perfect octave

Minor second

Major seventh

Major second

Minor seventh

Minor third

Major sixth

Major third

Minor sixth

Perfect fourth

Perfect fifth

Augmented fourth/diminished fifth/tritone

Augmented fourth/diminished fifth/tritone

Perfect fifth

Perfect fourth

Minor sixth

Major third

Major sixth

Minor third

Minor seventh

Major second

Major seventh

Minor second

Perfect octave

Perfect unison

Notice the one oddity of the chart: the tritone is its own inversion. This interval will become very important when we start looking at a very specific kind of chord, so hold onto it for a few more articles.

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