can G major sounds be used in an A minor song, and why..

[jancivil]

Any sound from anywhere can be used in any key or anywhere if you think it does something you like. I would advise to take prescription or proscription with a huge grain of salt at this juncture. There is no law.

In technical terms, there is one note native to G major that isn't part of "A natural minor", F#. If we form a scale now from A, the name for it is Dorian scale (note I didn't say 'mode' which exceeds our scope at the moment): A B C D E F# G.
But there is a form of minor, in *minor key* {"A Dorian" hasn't a key: it is not key of G major for example.} that contains our F# without calling it this other name: A {rising} melodic minor. (AKA Jazz minor in that context): A B C D E F# G#. (the melodic minor scale iin descension is the same as the "natural minor"; jazz minor hasn't this distinction).

[banjobounce]

I may be misreading your question, but is just A Dorian. G major is the parent scale. All of the diatonic triads built into G major could theoretically be used as a substitute chord or be superimposed over a minor, but generally the more applicable ones are D dominant sounds, c major7, f# half diminished, and em7( or 6). They all have common tones with Am.

[Starbright]

It all depends on how you look at it and how you decide to go:
1. you use added Intervals inyour chord, they basicaly follow the rule of minor and major 3/major and minor 3 so you don't get any avoid notes in your voicing.
2. It's a mode like A-Dorian (Key G-Major) : Am Bm C D Em F#m7b5 G
3. It's a borrowed chord from A-Dorian wich would mean it's a modal approach where you borrow chords from related scale For A-Minor: G maj7/Bm7/D 7 (A-Dorian), E7 (harmonic minor/secondary dominant), Bbmaj7/Gm7 (A-Phrygian) and so on...

[_leras]

No rules, but

The A minor scale is all the white notes, G major chord is also all white notes, so can easily be used with the A minor key/scale

If you change key to G major, then F is now sharp, so 'matching' chords would (mostly) need to change to use F# instead of F once you changed key.

Even between the two keys you have a number of chords that will be in both keys i.e. Am, G, C, Em, so very fine to switch between the two

Edit: added The to A minor scale, to make it clear that it's this particular scale in case misread as 'a minor scale has'

[RebJohn]

It’s actual quite simpler than all of this to get back to the OP’s question even though i know this is 2 months old. I’m a music professor and I just saw this and thought I would chime in to see if this would be helpful to you guys.

All you need to do to find out what chords will work in any key (major or minor) is simply build a triad (every other letter starting with the root) on every scale degree of the key you are in using the key signature. So…in the key of a natural minor - there are no flats and sharps - so the standard chords that will always work (no borrowing chords or anything fancy) would be - ace, bdf, ceg, dfa,egb, fac, gbd. Notice the last one is the g chord! That’s why it works.

If you were in the key of D major for instance which has two sharps (f# and c#), then the chords in that key would look like this

df#a, e,g,b, f#ac#, gbd, and so on…

And you might notice there is a pattern you can apply to make it easy then for every scale going forth

Capital Roman numerals are for major chords, lower case are minor, the 0 should be superscript and small but stands for diminished, + is augmented
For Major keys
I ii iii IV V vi vii0

For Natural Minor
i ii0 III iv v VI VII

For Harmonic Minor (when you raise the 7th note of the scale by a half step)
i ii0 III+ iv V VI vii0

By the way if you are using G major in the key of a minor you could see how you would be using the natural minor scale as you are using the scale as is. With the harmonic minor you would have a G# so of course you would not have a G major chord

Also as someone else mentioned knowing the relative key can be helpful. The relative major is three half steps up from the minor…so in this case C major. Relative keys share the same flats or sharps - so obviously C major just like A minor has no flats or sharps. Another useful thing to know is the parallel minor which is literally the same spelling. So A major is the parallel major to A minor. This is useful because you can borrow or “exchange” chords momentarily from the parallel key to make your piece sound interesting. Done right it works and sounds great.

I hope this all is helpful…have a blessed night!

[_leras]

It’s actual quite simpler than all of this to get back to the OP’s question even though i know this is 2 months old. I’m a music professor and I just saw this and thought I would chime in to see if this would be helpful to you guys.

I hope this all is helpful…have a blessed night!

Your answer is complete...

... but it is not simple and needs a lot of music theory to be understood.

Perhaps not your fault as music theory and naming have, imo, evolved to be much more complicated than they need to be.

It's great to highlight the shared notes, and chords, between different keys and scales.

(All these roads lead to Jerome Kern - All The Things You Are )

[jancivil]

Well, the OP did ask about basics such as how does the circle of fifths relate to their question, and there's the basic basics. I didn't get a sense it was time to do that much for someone new to concepts. There's no indication of their particular interests either, past that. But there's also people reading that aren't the original poster.
So as to basic basics, saying 'relative minor has the same key signature as its major counterpart' is correct, but eg., key of A minor that isn't altered for harmonic functional application, aka tonality - in a stricter than a colloquial sense - may not really carry 'minor key' more than it does "Aeolian"; ie., there's no V-I.

"music theory and naming have, imo, evolved to be" bla bla. Not at all, It's not changed for centuries (except we can't use functional harmony or modal practices or analysis for tone rows in dodecaphony or "atonal" serialism).

For example, the most surprising move in Wagner (AFAIK) "the Tristan chord", middle of the 19th c., can be described in a music theory course with exactly the same method you use for JS Bach, same vocabulary. Even at the juncture I would call just before the break from tonality, Schoenberg's Verklarte Nacht is still functional tonality. I'm not sure there was a model for a harmony with the 9th in the bass but figuring the bass there uses the same logic as figuring the bass in Bach, maybe there's one more number to care about.

[jancivil]

in the spirit of a simple[r] response:

what the circle of 5ths is for? Ive previously only used it for mixing in key while djing

It shows the closer in proximity to the further in proximity to a given key.
C to G adds one sharp; C to F adds one flat to C with zero flats or sharps. So your "G major sound" that isn't in A minor (same key sig as C) natively is that F#. (the concept then is: Add one sharp, or flat, you've moved one 5th in the circle in one direction or the other.)

At the other end of the spectrum, key of C# or key of Cb have respectively 7 sharps or 7 flats. We call this a distant key area (to "C" or a key with fewer sharps/flats). Now, another way to look at "C#" is by the name "Db", which you'll notice has 5 flats. Note that 7 + 5 = 12. Or Cb's enharmonic equivalent is B, w. 5 sharps.
The key with 6 sharps or 6 flats is G# of Ab. Also note that directly on the other side of the circle is our most-distant-to its opposite, a tritone (1/2 of the "octave"). {Tritone means three 'whole' tones: C, now D, E, F#.}

The term octave seems to confound, because conventionally diatonic seven note scales became the prevailing convention, despite an octave having 12 tones. This's not real intuitive.

[_leras]

So as to basic basics, saying 'relative minor has the same key signature as its major counterpart' is correct, but eg., key of A minor that isn't altered for harmonic functional application, aka tonality - in a stricter than a colloquial sense - may not really carry 'minor key' more than it does "Aeolian"; ie., there's no V-I.

"music theory and naming have, imo, evolved to be" bla bla. Not at all, It's not changed for centuries (except we can't use functional harmony or modal practices or analysis for tone rows in dodecaphony or "atonal" serialism).

Well this is a good example of what I was meaning by the 'theory' being too complicated.

Is it really minor or aelioan, or just the major where the melody starts on a different part of the scale?

That whole concept of modes, imo, can be greatly simplified. The naming for a start is hugely unnecessary and complicated. For most people that's just starting a melody on a different note.

(Not denying the vibe of different keys due to equal temperament changing, or the fact that exploring modes/melodies shifted in the scale produce some great and interesting results)

[AlainGeneva]

It's like Bb maj in C min.