Originally Posted by: ethanquarHow would you know if a song is in a minor key or major key, just by looking at the chords?[/quote]

That might not be enough information to know for certain. But in general you can look at the relationship between all the chords used & in which order. Frequently a song will start or end on the tonic chord. That can be one big clue.

But the deeper answer here is that you need to know the full context of the song. This includes all the chords used, which order & just as importantly the melody, which will tell you the scale being used. All of those together will help you identify the key signature.

I cover this in my collections on improv & music theory.

https://www.guitartricks.com/collection/music-theory

https://www.guitartricks.com/collection/learning-to-improvise

In particular there is a discussion of how to identify key signature in the first tutorial on improv that might help you with this specific issue.

Originally Posted by: ethanquarBut The chords of C major and A minor are identical right? So how would I know what key the song falls under?[/quote]

Yes, in the abstract they are. But this is like saying all English words use the same 26 letters from the same alphabet, so how can you tell one word from another? Or how can you tell what is a question or an declarative statement? Because it depends on the specific letters or words used in a specific order.

The answer is the same: you need the full context.

[quote=ethanquar]That would also bring up the question of why a relative minor needs to exist?

Relative minor needs to exist because it is a useful concept in music theory.

Often most of a song will be in a major key. But only one section (a pre-chorus, or bridge) is in relative minor for dramatic contrast. And often minor keys are harmonized differently so the V becomes major for a stronger leading tone.

Let's use the song 'Yesterday' as a practical example.

https://www.guitartricks.com/tutorial.php?input=1811

The song is in the key of G major. The beginning chord progression & melody makes this easy to hear & see.

But notice the little modulation in the 2nd measure of the verse:

F#m > B7

That leads to an E minor in the next measure. So we have this:

G (I) > F#m (ii of vi) > B7 (V of vi) > Em (vi) > C (IV) > D (V) then back to G (I)

Essentially this is just (I) > (vi) > (IV) > (V) with a fancy, dramatic way of getting from I to vi. And the melody is clearly on G major scale notes, emphasizing chord tones, changing to reflect the brief modulation.

Notice when we get to the bridge we start with the modulation that moves to Emin. And the melody emphasized the chord tones, leading to a sadder sound. So we can say that the bridge sounds like it's in relative minor. Because it starts with the modulation leading to the Emin chord, and eventually winds it's way back to G major.

F#m (ii of vi) > B7 (V of vi) > Em (vi) > C (IV) > Am (ii) > D (V) > G (I)

Now we can look at part of this as a brief change to relative minor! And importantly, notice how sad it sounds!

F#m (ii) > B7 (V) > Em (i)

And that simplifies the symbols in that section. Of course at some point we start looking at how it relates back to the home key of G major.

C (IV) > Am (ii) > D (V) > G (I)

Some songs have even more of an extended relative minor section. Other examples inlcude 'When I'm 64', 'We 3 Kings', 'Greensleeves'.

[quote=ethanquar]Like a 2-5-1 in E minor would be the same as a 7-3-6 progession in G major, so why?

Another more general answer is that it is more common to think of chords in terms of how they relate to the tonic chord. So in general it's more efficient to use 2-5-1, than to use 7-3-6. But again it always depends on the overall context of the song as determined by the chords, their progression order & the melody notes.

Relative minor (or major) is used when it helps to clarify the structure of the song to understand how the sections relate to each other.

Hope that helps!