Augmented fourth and diminished fifth

In the last episode we developed a table showing all interval names from 0 to 12 semitones. The only interval not named in that table is the six semitone interval. Here’s that part of the table:

5 Perfect Fourth c-f
6 ??? b-f, f-b
7 Perfect Fifths c-g

The question is: What name do we assign to the six semitone interval? One could have the idea to call the interval major fourth or minor fifth. For music theoretical reasons all perfect intervals (e.g fourth and fifth) are not separated into major and minor intervals. The reason for this will be explained in later episodes. For now let’s focus on the actual naming of the six semitone interval.

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Diminished and augmented intervals

Six semitones are one semitone less then a perfect fifth. This is the reason why that interval is called diminished fifth. At the same time six semitones are one semitone more then the perfect fourth. Thus this distance is also called augmented fourth. We get the following interval table:

5 Perfect Fourth c-f
6 Augmented Fourth f-b
Diminished Fifth b-f
7 Perfect Fifth c-g

Augmented fourth or diminished fifth?

Both the augmented fourth as well the diminished fifth have a width of six semitones. To distinguish between both we need to estimated the number of scale steps. For the key C-major we would simply count the white keys. In the case of the interval f-b we would get four scale steps namely f-g-a-b. Thus the interval f-b is an augmented fourth. In the case of the interval b-f we get five scale steps namely b-c-d-e-f. Therefore the interval b-f is a diminished fifth.

Next episode

In the next episode we will close the circle to the first episode: We are going to explain the complement interval using the key related melodic circle.

Summary

  1. Every diatonic scale has two different sized fourths and fifths. But these are not called “major” or “minor”.
  2. Instead these intervals are called perfect and augmented fourth and perfect and diminished fifth. To understand that, some deeper music theoretical knowledge is required which we’re going to provide in future blog posts.
  3. Augmented fourth as well diminished fifth have a distance of six semitones. To distinguish between both, scale steps have to be considered.

2 thoughts on “Augmented fourth and diminished fifth”

  1. Listen to this at night it’s mad scary yo I SWEAR! But Havock was a pure genuis but then the thing he used from Kitty With The Bent Frame- Quincy Jones that one sound and he said OH YES this is it LIKE TF!!! how do you hear that and say we’re gonna make a clasisc off of that

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