Interval inversions

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Continuing our discussion on the subject of intervals.

What happens when you turn an interval upside down? The correct musical term for this is inversion.

Example : C to G

E------------------------------
B------------------------------
G------------------0----------
D------------------------------
A--------3--------------------
E------------------------------

Is a perfect 5th interval

C D E F G
1 2 3 4 5

But G to C

E------------------------------
B--------------1--------------
G--------0--------------------
D------------------------------
A------------------------------
E------------------------------

Is a perfect 4th interval

G A B C
1 2 3 4

So although they contain the same notes these two intervals are named differently according to which way up they are.

Here are the rules about inversions of intervals:

Octave becomes unison
Unison becomes octave
Minor second becomes major seventh
Major seventh becomes minor second
Major second becomes minor seventh
Minor seventh becomes major second
Minor third becomes major sixth
Major sixth becomes minor third
Major third becomes minor sixth
Minor sixth becomes major third
Perfect fourth becomes perfect fifth
Perfect fifth becomes perfect fourth
Augmented fourth is unchanged when inverted
Diminished fifth is unchanged when inverted

Points to note:

Pairs of intervals and their inversions tend to add up to 9.
The sound quality of intervals and their inversions are the same
(ie: they are enharmonic)even though their tonality alters
(minor changes to major and vice versa).

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