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Music Theory Lesson 5 - Intervals - Part 3
Perfect Intervals 

The last thing I mentioned in the previous lesson (Lesson4 – Major & Minor Intervals), was that when inverted, major intervals become minor intervals. The Perfect interval, on the other hand, when inverted remains Perfect. 

Hold that thought! 

C – F (a 4th), is more specifically a perfect 4th, since F is in the key of C. If you lower the F 1/2 step to Fb , then the interval becomes a diminished 4th. 

C – G (a 5th), is more specifically a perfect 5th, since G is in the key of C. If you lower the G 1/2 step to Gb, then the interval becomes a diminished 5th. 

C – C is either a Perfect Unison (generally referred to as just “Unison”) or a Perfect Octave if the second note is 12 half steps higher than the first, (generally referred to as an “octave”). 

The key things to remember here: - 4ths, 5ths, unisons and octaves if *diatonic, are perfect 4ths, 5ths, unisons & octaves. - Perfect 4ths or 5ths compressed by 1/2 step are diminished 4ths & 5ths. (I suppose unisons & octaves technically are also, however I suspect that those spellings are a rarity). 

*diatonic…..meaning within a key B is diatonic to C, being the major 7th to C. Bb is not diatonic to C. 

NOW! Still holding that inverted perfect interval thought? As I said earlier, a Perfect interval inverted remains perfect. The reason this is true is because when inverted, Perfect intervals remain diatonic. 

Example: C – F (Perfect 4th, since F is diatonic to C) inverted – F – C (perfect 5th, since C is still diatonic to F) C – C speaks for itself. 

Three more things to remember about perfect intervals when inverted… 
1. 4ths become 5ths / 5ths become 4ths 
2. Octaves and unisons remain the same 
3. Perfect intervals remain perfect 

Lastly, the augmented interval. Any Major or Perfect interval stretched by 1/2 step is an augmented interval. 
1. C -D# (augmented 2nd) 
2. C – E# (augmented 3rd) 
3. C – F# (augmented 4th) 
4. C – G# (augmented 5th) etc…