Depends on the type of music you're talking about but using only notes and chords built from the major scale of the key would be the exception not the rule. I think the idea that the major scale is the "basic unit" of music and other notes are "borrowings" is harmful to the understanding of, say, rock music. In this sense, a small amount of "theory" is probably more harmful to compositional ability than none at all; trying to write rock music based on a basic understanding of traditional diatonic harmony is like trying to write Mozart with Guidonian hexachords. It just missed the point.

@Some_Guy - you're right, of course. I addressed the OP generally, and rock is but a small portion of music, (< 2 or 3%?), probably not coming under the term 'general'. I've played rock, etc. for many decades, and it doesn't follow general theory lines, especially regarding scale structure! Rock wouldn't work at all if it followed diatonic guidelines!

I picked rock music as an example not because it's a notable special case, but because 1) it's familiar to most of us 2) it's isn't usually thought of as high-brow/deviant/complex music 3) I'm currently writing an answer to a question of yours about chord function in rock music, so it's on my mind. My issue is with a sentence like ; "The majority of pieces will be in 'a key', and thus use the diatonic notes from that related scale to sound that way, but sometimes a twist in the journey will necessitate other notes (which by definition are from other keys/scales)", for 2 reasons (cont.)

1) that only using notes from the diatonic scale reflects the majority of pieces (however you want to characterise it geographically or time-wise: it's not even true for say the majority of western music from the last 100 years) 2) that any use of notes or chords outside of the major scale necessarily represents "borrowing" from another key or scale. I'm not saying learning about basic diatonic harmony isn't valuable: after all, you have to start somewhere. But the idea that this represents the "normal" rules which are "occasionally broken" is worse than useless to understand most music.

@Some_Guy - Would it not be better if you provided a separate answer here, as it seems I have so much wrong? It's a great opportunity to put it all right, and another aspect on the question will never go amiss.

the question is locked unfortunately! And sorry if I seemed overly critical

@Some_Guy - taking issue with 3.1: Just about every piece written for mainstream has a key sig. There would be no purpose for that is the basic tenet of using those diatonic notes wasn't foremost in the composer's mind. Take any number of pieces, and you'll most likely find that 75%+ of the notes contained therein are diatonic. 'Majority' to most folk is >50%, so even if it was that low, which I doubt, my statement stands.

I might have been a bit overenthusiastic with my criticism. I took your comment to mean that the majority of music uses only the diatonic notes, rather than that the majority of music uses mainly the diatonic notes. I'd like to address your other points but perhaps that'd be better in chat?

I don't want to seem nit picky but you've raised an interesting point in the last comment, so take this as a discussion rather than a criticism. I feel I was overly harsh with my initial comment, based on a misreading of what you were getting at

neverthe less the point about key signatures is an interesting one

Most mainstream music isn't written down at the time of composition to be fair. Folk traditions are generally aural traditions too. But the fact is that the conventions of the only reasonable orthographic system we have are fundamentally derived from common practice music

Personally, if I'm writing a transcription of a piece of music that used the flat 7 far more than the major 7th, I'm going to use the major key signature: because that's the convention we have: there isn't really a more convenient way of writing a pitch centre

it gets even weirder when notating music that is strongly modal, (without much interchange) but uses a scale based around a different set of notes

like if I was to transcribe this: open.spotify.com/track/1ymePDL4ftRHT2JnAZ54ku then I'd use a key signature with 3 sharps, because people looking at that will go "Ok, it's in E major"

when a more "faithful" representation of the modal nature of a song would have a single G sharp in the key signature (notice that this would produce a key signature with both a B flat AND a C sharp, and no F sharp in the piece was tranposed to A, MAD)

so you may as well just stick to the established convention and write with the established system of key signatures, even if that is derived from a system which no longer applies. After all, the English language's orthography is full of silent letters and inconsistent vowel transcriptions: many with an etymological reason behind them but many because of what the grammarians thought was the "correct" derivation of the word despite it actually not having any linguistic merit.

And yet it's still perfectly adequate to write these sentences to you despite its anachronistic aspects and often plain senselessness

Just as an aside by the way, I really respect your contributions to this site so I really hope I haven't pissed you off here!

Also, I'm eager to here your thoughts on the answer I posted about bVII chords when you have some time!!! :)

I spent more time on it that I should have, probably, but I really ended up going "down the rabbit hole" so to speak.

Interesting thoughts! Incidentally, E has 4# - it's A with 3#, I'm sure you know! Yes, I often have arguments with myself about key sigs for modal stuff - while they are obviously technically correct, and probably so historically, they don't really convey the right message. Look at one # and you think Gmaj., until you spot some D# when it becomes possibly Em. But with that one sharp I certainly wouldn't be thinking oh, it's going to be A Dorian...

No, I didn't get pissed off, as you appear to be pretty knowledgable, but I try to make answers as unequivocal as I can, but sometimes in a rush there's odd things creep in that get misconstrued - especially when not read and digested well... and thanks for the praise. we have lost a couple of really good contributors lately, I guess due to unnecessary comments, but my skin is nearly as thick as I am. Did that come over right? Haven't had chance to peruse the b7 bit yet, but surely you'll get

my thoughts at some juncture.

I don't really tend to follow the behind the scenes stuff at this site to be honest. Who've we lost?

One of the things I've always lamented is the lack of a PM system here, there have been so many times when I've really felt like just dropping someone a message either to say thanks, or because there are some really interesting musical minds around here!

I just think it's a great thing that there are people out there willing to put their time and effort into freely sharing their knowledge, I'm often left thinking "who ARE these people" haha

although as "some guy" I suppose I can't exactly criticise for that now can I?

@Tim I think I have to write @TIM to ping you here, so here we are

@Tim 4 indeed! woops!

Modulation to other keys is what makes music interesting. Music usually modulates briefly to other keys, then modulates back. Or, it may modulate for a more extended time.