8: Interval Quality

Intervals, as we said in a previous article, are the sizes of the spaces between notes. Each interval is uniquely identified by two things: a quality which comes first and a size which comes second.

You’ve already seen the size of an interval and how to calculate it.

A B C# D E F# is some kind of sixth, since we’ve spelled out six distinct letter names. The quality, perhaps, is trickier. Depending on the interval, it may be any one of the following: perfect, diminished, augmented, major, or minor.

Here are the rules.

·       If you can spell out a major scale and a minor scale and the interval is the exact same in both, then the interval is perfect

·       If the interval only appears in the major scale, then the interval is major

·       If the interval only appears in the minor scale, then the interval is minor (except the minor second, which is the distance between any two consecutive/adjacent notes, like E/F, B/C, A/Bb—which is not the interval between the first and second degres of the minor scale)

·       If the interval is one half-step smaller than a minor or perfect interval, then it is diminished

·       If the interval is one half-step larger than a major or perfect interval, then it is augmented.

Let’s go through one example of each. I will assume you know how to go through the process of spelling scales, so I will just spell them in one go, without explaining each step.

First a perfect fourth:
C major is C D E F G A B C
C minor is C D Eb F G Ab Bb C
The fourth between C and F is perfect, because when we look at C major and C minor, they both have an F natural in their fourth degrees.

Now, a major third:
A major is A B C# D E F# G# A
A minor is A B C D E F G A
The third between A and C# is major because in the scales rooted on A, the third is only C# in the major scale.
 
Followed by a minor third:
B major is B C# D# E F# G# A# B
B minor is B C# D E F# G A B
The third between B and D is minor because in the scales rooted on B, we the third is only D in the minor scale.

Now a diminished fifth:
F major is F G A Bb C D E F
F minor is F G Ab Bb C Db Eb F
Ordinarily, F to C would be a perfect fifth—it’s the same in both major and minor. So if we now shrink it by a half step, keeping the F static and moving the C down to Cb, we now have the distance of F to Cb as one half-step smaller than the distance from F to C. We’ve just made a perfect interval one half-step smaller, so it’s diminished.

Similarly, we have
A minor is A B C D E F G A
The sixth between A and F is minor, so if we keep A where it is, and flatten F by a half step, and then consider the distance from A to Fb, we’ll have shrunk a perfect interval by a half step, and therefore A to Fb is a diminished sixth.

And finally an augmented third:
In C major, we have C D E F G A B C
The third between C and E is major, so if we fix C and move E by one half step to E#, we have now expanded a major interval by one half step, and therefore C to E# is an augmented third.

Similarly, we have
D major is D E F# G A B C# D
The fourth between D and G is perfect, so let’s fix D and sharpen G by a half-step to G#.
If we do this, by expanding a perfect interval by a half-step, we’ve now created an augmented fourth.
 
The minor second (the half step—the distance, say, between E and F, or B and C) is the smallest intervallic unit in Western music. All other intervals are built as multiples of them.

Once you’ve determined the size of the interval and its quality, you have all the information required to name it: simply place the quality before the size.

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