#1
I know a lot about music theory but ive never really known how to tell if it is in a major or minor key or a mode. Help please?
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#2
if it is minor it will usually sound sad. major; happy.
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#3
Well, if something is in a major or a minor key the first things I look for is what is in the key signature. If I'm in the key C major (no sharps of flats) and the first chord and the last chord includes a C in the bass then I can assume that it's in C major. However, if the key signature still sports no sharps or flats and the piece starts and ends on a chord what an A bass then the key is mostly likely to be A minor, C major's relative minor. The A in the root of the tonic chord and the raised 7th (G#) are dead giveaways. That's just one example though.
#4
Quote by Banjocal
if it is minor it will usually sound sad. major; happy.


No.

Quote by UnmagicMushroom
[...]the first chord and the last chord includes a C in the bass then I can assume that it's in C major.


Also, no.

Minors sounding sad and majors sounding happy is a short-sighted misconception. Remember, it's not about the notes you use, but how you use them. The first or last chord of a piece or phrase has no bearing on the tonality of the piece. However, it is true that a fair majority of pieces are written like this--and most pieces end on either the tonic or the dominant--this is not an effective way to find the key.

You have to listen to the music (duh; ) the chord that sounds the most resolved is going to be the tonal center of the piece. Practice on 3-4 chord songs. See if you can pick out the resolutions.

You can also look at cadential patterns. If you're not familiar with cadences, now's the time to learn. Be wary, though. This needs to take a back seat to hearing resolution: chord sequences are not always as they seem. For example: A - D - E. Probably in the key of A, right? I - IV - V? Sometimes you hear IV - bVII - I. Or V - I - II. All three of these effectively establish a major tonality, but achieve it through different means.
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#5
soviet_ska, those methods mightn't be foolproof, but they are helpful. If you see a key signature with no sharps or flats and it ends on a C, I'd be very surprised to find out it's in A minor. Some give aways for minor keys are the raised 6th and 7th (especially in classical music).

Listening for where it resolves is the most effective way, unless you modulated somewhere without realising.
#6
Quote by Jesse Clarkson
soviet_ska, those methods mightn't be foolproof, but they are helpful. If you see a key signature with no sharps or flats and it ends on a C, I'd be very surprised to find out it's in A minor. Some give aways for minor keys are the raised 6th and 7th (especially in classical music).


TS never said he had sheet music for the songs he's trying to determine the key of. Seeing that he has "Crazy Train" and "Back in Black" in his profile, I'm judging for at least some of the music he is playing he won't be able to see a key signature.

I agree that, realistically, looking at the first or last (preferably last) chord of a piece is a decent method of determining the key, it is not 100%. Furthermore, the position of a chord in a phrase has no bearing on it's function--the sooner we all get that in our heads, the better. If you can hear what chord is the tonic, all you have to do is look up the chord that is in that position.
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#7
Quote by Corvette_cav
I know a lot about music theory but ive never really known how to tell if it is in a major or minor key or a mode. Help please?


The short answer is that you don't know as much about theory as you think you do if you're "knowledge" is ahead of your ear.

Theory is only relevant to the extent that it's related to things that you can process as you hear them. Otherwise it's like saying that you've mastered german grammar but you don't know how anything is pronounced, and you don't know any words. It's meaningless.

I recommend this book a lot, but, well, here it goes again: Ear Training, by Keith Wyatt, et al.

That book includes a tremendous amount of theory, but it introduces you to the theory through the process of teaching you how to hear it.

Quote by Jesse
unless you modulated somewhere without realising.


In that case, you need to learn how to hear the modulation. Because, ultimately, in that situation looking at the last chord doesn't help you, either.