Re: Perfect fifths... a question

[xvimbi]

I have seen people refer to the tritone as a diminished fifth, rather than a superfluous or augmented fourth. I don't know if there are circumstances that make it compelling to use one name over the other, though. To avoid ambiguities, you might just as well use the word "tritone" for the interval of three whole steps and not worry about the other names.

[allchopin]

A 5th cannot be called minor because it does retain the title of perfect.  The intervals taht cannot be called 'minor' are the unison (same note), perfect 4th, perfect 5th, and the octave.  Instead, they are called diminished.  However, the 2nd, 3rd, 6th, and 7th, can be called minor when the bottom note is lowered by a half step.  And finally, when raising the upper note in any type of interval, it is called augmented.
The tritone is so called because back in the day when music was more deeply tied to religion and the church, the interval of 3 whole steps was considered evil and devilish (because of the sever dissonance).  
I hope this is plain English.

[steve]

And in the blues scale it's called a Flat Five.  Thought I'd throw that in  

[allchopin]

Basically  

[Rach3]

C-Gb  <-- diminished fifth
C-F#  <-- augmented fourth

A TT is written as a 'fifth' usually when it tends to contract towards the third, and as a 'fourth' when it tends to expand to the sixth, thus the tritone in the V7 is a diminshed fifth (the 7th degree being diminshed from the leading tone of V). Likewise, an german augmented sixth has an augmented fourth.

As to why it isnt called a 'minor' 5th, its terminology: if there are two possibilities, they are major and minor; if there are one [three] posibilities, they are perfect [and diminished and augmented].

[Nightscape]

Try this on the piano and you'll see why some intervals
are called "perfect":

Play a C major triad with your left hand, then play a C and a G (open fifth) an octave higher with your right hand - it sounds fine, right?

Now play a C minor triad with your left hand and repeat what you did with your right hand - it still sounds okay.

This is why the perfect fifth is called "perfect" - it works in either a major or minor setting.  It is a somewhat of an ambiguous interval. That is also why there is no such thing as a major fifth or a minor fifth, since the fifth does not lean towards either by nature.  

As far as the tritone is concerned, it has two names: the augmented fourth and the diminished fifth.  There is no difference in sound between the two, the only difference is how they LOOK on paper.  For example, an augmented fourth on paper might be a C followed by an F sharp - the appropriate spaces and lines on the staff would be used to give the "look" of a fourth.  A diminshed fifth "looks" like a larger interval on the staff paper, but in our system of tuning, there is no difference in sound (in the above case, F sharp and G flat are enharmonic).

[Rach3]

More to the point, the perfect intervals are the most consonant; major/minor intervals like thirds and sixths are relatively dissonant.

[joeltr888]

augmented 4th = tritone

diminished 5th = not a tritone

The tritone means (quite obviously) 3 tones. Think C to D to E to F#. The dim5, (C to D to E to F to Gb) is two whole tones and two half tones... not a tritone. They sound the same in isolation, but functionally they're two different things. Just had to clear that up.

I'm not sure about the exact origin of the naming conventions, but they seem to make sense with a little thought. Perfect intervals are of a different type - they occur early in the harmonic series and are thus a great deal more transparent... this accounts for the name "perfect". This can probably be traced back the middle ages/renaissance when consonance/dissonance were still being argued, and the perfect intervals were acknowledged as just that. The major intervals were probably named so for their position in the major scale, and the minor intervals accordingly (I'm not sure about the m2). This kind of explains why perfect intervals then become dimished/augmented... they remain perfect in both the major and minor scales... diminished and augmented basically imply a functional alteration of the scale tone altogether, not just a "majoring" or "minoring" as it were. When a tone is major or minor, it retains its functional position within the scale/key.

Of course, I could be completely wrong...