Wondering about the names of the other intervals found between the notes of the Major Scale? Meet Minor, Augmented, and Diminished!
View the full lesson at Chromatic Intervals | JustinGuitar
Hope this summary helps:
Perfect intervals (i, IV, V, octave)
Major intervals (ii,iii,VI,VII)
Perfect interval → Raise by 1 semitone → ‘Augmented’
Major interval → Raise by 1 semitone → ‘Augmented’
Perfect interval → Lower by 1 semitone → ‘Diminished’
Major interval → Lower by 1 semitone → ‘Minor’ → Lower by 1 semitone → ‘Diminished’
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After all this time, this lesson helped me understand the diatonic vs. chromatic distinction once and for all.
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Thanks @Earthless … very useful summary.
For something longer, you may like this: Intervals, scale degrees and more
I didn’t get: why should a perfect interval minus a semitone and a major interval minus two semitones be named in the same way (i.e. diminished)? I don’t get the reason
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Hi Francesco,
This kind of diminished intervals (major → minor → dim) come into play in case of double flats. Double flats/sharps always have more straightforward enharmonic equivalents, but they are used to keep consistency when analyzing a key (i.e letters are not repeated within the key).
As for why not another term is used… I guess it’s due to tradition.
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It might help you to think of Major and minor triads.
Let’s start with a C major triad. It contains two internal intervals - a major 3rd followed by a minor 3rd. Its overall span is a perfect 5th interval.
Diagram 1
The major interval spans four semitones.
The minor interval spans three semitones.
The perfect interval spans seven semitones.
Why is it called perfect? I am unsure - perhaps there is something good / right / balanced / just-so about the fact of having one each of major and minor within.
Lowering the major 3rd interval by one semitone, inverts the internal intervals whilst maintaining the perfect nature of the 5th. A C minor triad results.
Diagram 2
Lowering the major third makes all the difference there is between a major triad and a minor triad. Which came first - naming the intervals major / minor or naming the triads? I am unsure. But, imho, it would not make sense if the intervals and triad types had different names.
Returning to the staring point of the C major triad, if the second of the internal intervals, the minor 3rd, is lowered by one semitone it will become a diminished 3rd. Also, the overall span of the perfect 5th will be lowered by one semitone, meaning we need to change the analysis again. The major triad becomes something a little peculiar - a major b5 triad. Its overall span is now a diminished 5th.
Diagram 3
If we similarly start with the C minor triad and lower the second of the internal intervals, what was a major 3rd becomes a minor 3rd. this new triad therefore contains two minor 3rds. Its overall span, as above, is now just six semitones making it a diminished 5th. This has created a diminished triad. Unlike the major b5, these are more common triads.
Diagram 4
From the starting structure of the major triad, we saw that lowering the (first) major interval resulted in a minor interval (and a minor triad). If we take another logical step down that path - lowering that same interval by one further semitone, we will create the following:
Diagram 5
There is a a diminished 3rd in the first instance (we have already seen that a two semitone interval formed by lowering a minor 3rd is named a diminished 3rd). The second internal interval is stretched wider, it alters from a major 3rd to become an augmented 3rd. The overall span remains unchanged - a perfect 5th.
This is an even more peculiar triad than the major b5. In sound, and practicality, those three intervals and notes would ordinarily be named and assigned differently as a Root, 2nd and 5th making for a sus2 triad.
I hope that helps.
Cheers 
| Richard_close2u | JustinGuitar Moderator, Guide & Approved Teacher
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I never considered this perspective of taking intervals “inside” the triad.
Then, my interpretation would be: you call “diminished” when a minor interval is lower by a semitone. This means that the perfect interval, which we may think as formed by a major and a minor interval, will be called as well diminished because we can think that the minor interval “inside” is lowered by a semitone (and so, must be called diminished)
I don’t know if I explained myself/makes sense
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