This is a music thread, and for those of us who don't really want to just talk about what we're listening to or bands, lets talk. The 5m in a major key. What do you think?

I'm afraid I'm slightly confused about what you mean by a "minor fifth."

If you are talking about the interval of a fifth, it can never be minor; it can only be perfect, diminished, or augmented. So, if you mean a diminished fifth (d5), that has plenty of uses in a Major key. It is mainly used in the V7 chord or viio7 chord in which it resolves inwards towards the tonic triad, providing strong direction back to I and completing the movement through the phrase model. It can also be used in common tone seventh chords, of course, but those are just a matter of prolonging a chord anyways, so they're not very interesting from a functional standpoint, and there's no difference between using those in a Major key and a minor key.

If you mean a minor five chord (v), then I'd have to say that this has absolutely no business in a Major key. The dominant function is extremely important in tonal music, and when you make the chord built on scale degree 5 minor, you ruin the phrase model, because there is no leading tone to direct the chord back to the tonic. The only way I can image one using a minor v chord is in some sort of modal mixture intended to foreshadow and set up a modulation that's about to happen. It would be very odd to introduce a modulation through a minor v chord, though; it's much easier to point to a new tonic by introducing a new V7 or even viio7 and resolving that to the tonic. Furthermore, pivoting to a new key is impossible unless the two keys share the chord (in root and quality) that is used as the pivot. A minor v chord in the original key is impossible, so it can't be used as a pivot.

So, if you want to have a discussion about a "minor fifth," you will need to clarify what you mean by this phrase. If you mean a diminished 5th interval (d5), it's quite commonplace and not all that interesting, although it can be used in modulations, making it slightly more interesting. If you mean a minor five chord (v), it's nonsensical in tonal music.

You're right, I should've clarified between the 5th interval and the five chord. For this I mean the minor five.

On the contrary though, I think the minor V can work perfectly well in tonal music in a major key. Minor V, IV, I, I seems to work for to my ear at the very least.

Start with a progression of V, VI, IV, I, play it twice, then move to minor V, IV, I, I and tell me what you think. I think it sounds fine.

I agree with parts of both, I played around with v as an expansion of the tonic (I v I) with some good results. This may not make much sense in the context of basic music theory, however, as a form of modal mixture, I find it to be perfectly valid. Modal mixture, while chromatic in nature, is still a part of tonal music.(The Complete Musician 3rd Ed. by Steven Laitz PP. 433) As far as its use as a pivot chord, modal mixture is not exclusively, or even mainly used as a modular technique, although it is frequently used as such. The most common use is as an harmonic embellishment. As for the minor v as modal mixture, it is certainly not common, but then again, neither is putting a chorus in a symphony, but that seems to have worked out well enough for Beethoven.It could also be a cool harmonic weirdness from the hypomixolydian or dorian mode, but I have not really studied modal music. I am interested to hear everyone's insights on this.