Perfect to diminished interval confusion

victorludorum

I read that if we make a perfect interval a semitone lower, we have a diminished, but I'm a bit confused when I look at the C Major scale. If C to D is a major, C to E is major and C to F is perfect, If we remove a semitone we get back to C to E, which is a major interval, rather than a diminished one. What am I missing here? Is it simply that I should be considering it as C to Fflat, even though Fflat is the same as E?

jasonbone75

You are looking at the fourth. Do it on the Fifth i.e. G down to F# and that is the diminished (I'm a bit of a caveman on theory though so others will be more eloquent and accurate!)

Edit: and yes on further thinking a diminished 4th and Major 3rd are harmonically the same note just one is contextually a 2 tone up from root and the other is a semitone down from 4th.

viz

Yep, C to Fb is a diminished 4th. Diminished 4ths are most commonly seen not between the 1 and the (dim)4, but between the major 7th and the minor 3rd, for example, in A minor, between the G# (the leading tone) and the C above (the minor 3rd). You can imagine playing the two notes together, then playing B, then finalising on A. 

digitalkettle

I'm not sure how practical* this all is but...

• If you drop a perfect interval (unison, 4th, 5th) by a semitone, it is diminished
• If you drop a major interval (2nd, 3rd, 6th, 7th) by a semitone, it is minor
• If you drop a minor interval by a semitone, it is diminished

Examples:

• maj 2nd - C to D (2 semitones)
• min 2nd - C to Db (1 semitone)
• dim 2nd - C to Dbb (0 semitones!)

* guess that's why we're in the Theory section

digitalkettle

In a diminished 7 chord...let's build one on C# to start a cheesy jazz progression (Cmaj7 -> C#dim7 -> Dmin7)...you've got:

• root - min3 - dim5 - dim7
• = 1 - b3 - b5 - bb7
• = C# - E - G - Bb

If you built a diminished chord on a different root, such as Db, the note names would be:

• Db - Fb - Abb - Cbb