Interval Quality
The first word in describing an interval is one of these five: Major, Minor, Perfect, Augmented, Diminished. Each quality may be abbreviated as follows:

M = major, m = minor, P = perfect,  + augmented,  diminished
Major intervals are 1/2-step larger than minor intervals. Augmented intervals are 1/2-step larger than major or perfect intervals. Diminished intervals are 1/2-step smaller than minor or perfect intervals.
The quality of an interval depends on its numeric value. Any interval may be diminished or augmented but only 2nds, 3rds, 6ths, and 7ths may be minor or major. Likewise unisons, octaves, 4ths, and 5ths are the only intervals that can be perfect.

13 EQUALS 6?
Intervals over an octave share the same qualities as those an octave smaller. To reduce an interval by an octave, subtract 7. A 12th can be perfect since it reduces to a 5th.
The interval of 1/2-octave is often called a tritone.
From fa up to ti it is an augmented fourth (+4).
Inverted (ti up to fa) it is a diminished fifth (-5).

Intervals Top

The intervals up from do of the major scale are all major or perfect.

The intervals down from do are all minor or perfect.

To invert an interval, place the bottom note on top! A perfect interval remains perfect when inverted. A major interval becomes a minor interval and vice versa. The numbers always add up to nine!
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