Yeah, not relevant, I decline.
Terabytes of text have been spilled on this exact subject. Were you not aware of this, or are you asking me to narrate Google for you?
This is demonstrably false with a tuner that shows frequency, unless you believe you’re revealing something new by explaining what octaves are.
Oh, I think we’ve mostly conceded that the thread has completely wandered off. Part of the reason is, I think, this question is as broad as asking why someone has a mental block with math or understanding grammar. I don’t think any of us can answer why. I mean, we could try to teach here, but it’s such a broad subject and very difficult to understand where the OP is getting hung up. Hence many folks’ advice to simply start as simple as possible, with the most basic theory book. If you really want to learn theory (and I don’t think it’s necessary at all, but it is helpful for many aspects of music) you have to start at the very most basic concepts.
It’s not clear to me what the OP knows, if he even knows what a major scale is, and there seems to be some difficulty in understanding intervals, so it’s difficult to tell them where to start other than the very beginning.
But I’ll try one theory 101 lesson on intervals. Do we at least know the major scale? The basic one we start with is CDEFGAB and then back to a higher C, (which people usually sing as “do re mi fa so la ti do.”) Take a group of three consecutive notes: CDE. The first note (C) is a “first” (actually called a “unison”) with itself. The second note (D) is a second from the C. The third note (E) is a third from the C. And so on. Let’s start with G and take five notes (we’ll need to wrap around the scale to the beginning): GABCD. The D is the fifth note in the sequence starting with G, so it is a fifth from G. What is A in relation to G? Well, it’s the second note, so it’s a second.
Does that make sense? There are other complications like major, minor, augmented, diminished, but let’s not worry about that right now. That’s all you have to know at the moment to count out intervals.
What’s the relationship of D to A? We’ll let’s count: DEFGA. That’s five. D is a fifth below A. A is a fifth above D.
Does that make sense? If so, we can move on to how triads, the basic building block of harmony/chords, are built based on this information. If not, what are your questions?
Here’s a little quiz, using our C major scale, and ignoring major/minor/etc for now:
OP ignore this side discussion:
What JKellyMap is objecting to is that a II chord is major. A ii chord is minor. So a I-II progression is somewhat unusual (and why he’s calling it a V-of-V. D is the V chord of G, the V of C. Dm is the ii of C, not the II of C.) In Roman numeral notation the convention is that capitals indicate major, and lowercase letters indicate minor.
Please, OP, I hope you took my advice and ignored that. 
So what’s up, pohjonen? Did we come anywhere close to answering your questions?
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I think we scared those people off three pages ago.
“fundamentally helped get their tone”
“felt they sounded different” and
“thought they sounded and played better” are all pretty much saying “I don’t know why they did it”
But Jimi, for the one that I mentioned, very significantly did it because it helped him sing in tune. Lots of rockers tune down as they get older. These musicians didn’t change their repertoire based on the identity of key centers. They just made the change, and it didn’t matter to anyone listening. It doesn’t even matter why they did it.
If you don’t want to do thought experiments of mine, maybe you can interpret this post, because I can’t make head or tail of it. (Using the word “arbitrary” doesn’t move a thought forward unless it’s in a context.)
Well, no. Tuning down a guitar definitely makes a difference in the string tension, which affects how the instruments feels & strings bend, the gauge of strings you can use, etc. There’s plenty of reason to choose to tune down that affects the sound and the way your instrument plays. I’m not a guitar player, but that much seems obvious to me, just like how certain keys play different ways on the piano because of the way they fall under your fingers.
Thank you, **pulykamell **- you are far more patient than me. You are correct.
Isn’t there a distinction between “fifth scale degree” and “interval of the fifth”?
In C Major, “the fifth” is G, the fifth scale degree.
In C Major, “D and A are fifth apart” (interval)
Note that not all notes separated by 5 notes (counting the start note as 1) are intervals of the fifth. In C, B up to F is 5 notes just like D,A but the B,F interval is a (pick one) augmented fourth, dimished fifth or Tritone.
There is similar confusion by naming chords that way. That’s why we say “the five chord” or better, “the dominent chord”. Of course, in context we usually know we are naming chords.
Yes. If we were starting the system from scratch, we might want to forget about “whole steps” and just think of an octave as having TWELVE notes (rather than seven). The “half-steps,” in other words. Call them just “1” (C? sure, why not) through “12” (B). Then all your “five” intervals would be the same (but we’d call them “eights,” if we stuck with the “start on 1” principle – or better yet, have the new system start on zero, so we’d call these “sevens”).
Hey! I specifically said let’s ignore minor/major/diminished/augmented for a reason! I didn’t say all the fifths need be perfect fifths. B to F is still a fifth – a diminished one. (And, yes, it depends on context; it could be an augmented fourth).
But I wanted to see if the OP can first glom onto the basic concept of scale degrees and interval naming. We can tidy things up later.
pulykamell - gotcha.
And my assumption that referring to a “fifth” implies only a Perfect Fifth is an example of sloppiness that I was trying to avoid.
I’m going down to the liquor store to buy a fifth 
I was a little uncomfortable, too, knowing that “a fifth,” when unqualified, refers only to perfect fifths, and we also have the stickler of F-to-B being an augmented fourth, but we can make that the next lesson. I was just hoping this might spark a little “a ha!” moment of what an interval of a third means, and why it’s called a third, and what it means to say two notes are a third apart, before getting into the nitty gritty with whole steps and half steps. Or we could do the next lesson on triads, and how basic Western harmony is based on stacking thirds before getting into whole step vs half step, major vs minor.
Piano, as mentioned above, really does help with this visually, although we can continue our explanations with guitar, as well. It’s just a little more difficult as the pattern of notes doesn’t just repeat one octave after another.
“fundamentally helped get their tone”
“felt they sounded different” and
“thought they sounded and played better” are all pretty much saying “I don’t know why they did it”
I repeat: Those sentences are very uninformative on the topic, and not responsive to the point.
I know that the tension changes and that can affect people’s choices. But as to why they do it, there may be a few reasons. One of them is to sing lower, as it most surely was for Hendrix.
But the principle here is made, that none of these musicians made that call based on pitch values, and in fact that that made no difference in the decision except to overcome vocal limitations, or wanting more facility. It’s only pragmatic, and not musical.
I can’t help but note that the OP is very broad, but expresses confusion. But people are confused about theory at the start, and way up into higher studies, so we can’t pin down the level from the OP title.
I found it very informative to realize, a lot later than I should have, that the notes repeat after 12 of them and that there are numbers and letters to indicate the sequences of keys. This means that every 12th note on the piano is the same note. Every 12th key on a guitar are the same notes. Maybe it doesn’t sound profound to someone else but it was a great comfort to me.
I know that if a song is in G, that regardless of the register, I’m going to be singing the same song. I also find it helpful to think visually about keys, chords, fretboards and everything else. It seems to help me move around musically. When the keys are lettered it’s easier.
Maybe I was missing the obvious. It’s great when obvious things fall into place, and sometimes it isn’t clear why you didn’t understand everything right from the start. but that’s not the way it works for me anyway.
YMMV?
Every 13th note is the start of the repeat actually. There are 12 semi-tones in a scale, and you don’t see the repetition until the 13th note. Even on a guitar, the 12th fret is a repetition of an open string (0), not the note at the first fret. Nitpicky perhaps, but true.
Yes you got it, I didn’t. The twelfth fret is a repeat of the note at the nut, or the zero fret on some gutars, so there are 12 unique pitches.
I think that many things about this are “obvious”, but the progress one makes is to somehow connect many obvious things in a whole, naturally. I never thought about this stuff for decades after beginning to play btw, so I have religion about it.
But for a confused beginner I think there are some undeniable fundamental facts you might want to know: that the twelve pitches just repeat, That each one may be a key center for a song, the analog between the frets and the notes on a piano, the makeup of a chord, and the variations, and most importantly, tonal or key centers as heard in songs that you can hear as a listener.
To me the most important tool for that is the ordering of things for mnemonic purposes.
Good points, but for a confused beginner (who seems to have vanished), I’d say playing a song or even a riff, like Slow Talkin’ Walter will bring more satisfaction. If I had to learn all the theory before I picked up a guitar, I’d have quit. The theory made more sense in retrospect after I had a few tunes under my belt.
Well, I got the impression that the OP knew how to play a good number of tunes and was interested in learning theory for its own sake. But that’s another way to approach it. If the OP comes in and gives us the names of a number of songs he or she knows, we can help to break down some of the theory using those songs as teaching examples.