Perfect 4th 5th...?

[Tapout73]

Can anyone explain to a begginer what a perfect 4th and perfect 5th are in relation to intervals?

[dlloyd]

Play a plain old major scale.

The fourth and fifth notes are a perfect fourth and perfect fifth away (respectively) from the first note.

If you want to know why they are called 'perfect' rather than 'major' it is down to the way they behave in inversion (when the lower note is raised an octave)... major intervals invert to minor intervals, but perfect intervals invert to perfect intervals.

[gre107]

Perfect 4ths and 5ths are just what we call unaltered 4ths or 5ths. It really is that simple.

4ths and 5ths in diatonic chords (or better yet harmonies) are always the same distance from the root. If you sharpen either they are called augmented and flatened they are called diminished.

A 4th or 5th really have no meaning for an unaltered chord structure. That is to say they do not change the color of a chord unless they are different from their respective interval. For example: if you play a minor chord and add the fifth or remove it the chords sound really does not change. Now on the other hand by altering the 3rd or seventh of the chord you will create a minor or major chord and alter the sound of the chord dramatically.

Chords are built on invervals of "stacked" thirds and the 5th is always the same for a major or minor chord. E.g. Cmaj = C E G Cmin= C Eb G. The distance from the root C to the 5th G is always the same. Hence why we call them perfect.

I would recomend sitting down at a piano and studying the intervals this way along with a good beginners theory book. It will all make sense then.

Well, I hope this has helped?

All the best,

Gerry

[mutedeity]

Ahh, one of my favourite subjects. Why is it called a perfect 5th. gre107's definition is pretty spot on there. On the other hand I personally will and do argue the point that the reasoning behind calling either perfect is arbitrary, and more based on paradigm than any kind of intrisic reasoning.

The arguments are that P5ths are either unaffected by inversions or they remain the same interval from the tonic whether they are major or minor. My answer to that is that the only interval that really remains the same in a mathematical sense is the tritone. If you look at the interval it is always 6 semitones from the tonic either ascending or descending. A "P5" is 7 semitones ascending but 5 semitones descending.

In diatonic harmony it is true that the addition or ommision of the "P5" does not affect the functionality of a given chord, for example a dominant without the 5th will still have the same cadenical context as it would with one. On the other hand, the inversion of a "P5", a "P4" will affect the function of a chord greatly. So in this sense I find it hard to see what is intrinsically perfect about either interval.

Still, most people refer the these as P5th and P4th intervals and it is academically correct to refer to them as such. The simple answer is that the P4th and P5th are enharmonic respectively to the 4th and 5th degrees of the Major scale.

[hunta]

They are PERFECT intervals. That's why so many famous rock songs are I IV V progressions.

[BassChuck]

There's a pretty good chance that the term "perfect" for 4th's 5th's and octave came from ancient church music when it was, by church law that music could end only octaves and 5ths. (the 4th is the inversion of the 5th and so... perfect also).

What is really interesting is that the octave and 5th are the intervals that are easiest to tune and were therefore considered 'consonant' early on. As music progressed composers, for a number of different reasons, used intervals that were closer and closer together. Our ears have accepted this and we now find the sound of major and minor 3rds to be 'consonant'. We even tolerate 2nd's in a lot of musical situations as consonant. But the 4th is still a rather unfriendly interval and considered to be disonant by quite a few people. It has to do with the overtones not lining up together the same way they do with 5ths and octaves.. 4ths don't have that pure sound that 5ths and octaves have. Try a chord built totally on 5ths (C-G-D-A-E) and compare that with a chord built on 4ths )C-F-Bb-Eb-Ab). They really have quite a different quality.

[gre107]

Thanks Mutediety and BassChuck! Those are the two additional areas I did not want to get into! I wasn't nearly awake enough (lack of caffine... Ahem) this morning to try to verbalize anything but the rudiments. Now all I have to do is find my keys... hmmm

Peace!

[DocBop]

Originally Posted by hunta

They are PERFECT intervals. That's why so many famous rock songs are I IV V progressions.

That is chords not intervals. That is called Tonic, Subdominant and Dominant chords.

[Tapout73]

Interesting responses...Thanks everyone!