#1
Help the music theory n00b!

How does one identify the key of a piece of music (if one knows how to play it or has the notation)?

I've been told there's no such thing as "the key of E minor". How would something that sounds minor in the key of E be referred to?

If the piece of music is based around a mode, let's say Myxolydian, rooted on G, would that be in the key of G or in the key of C (as C is the relative Ionian mode)?

Please explain. I've heard a lot about keys, and was expected to identify them in music class, but was utterly confused.
#2
Em is G major
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#3
Of course there's such a thing as key of Em.

You can tell what key it is by the flats/sharps that are being used.

For example, C major and its relative minor (Am) have no sharps or flats. You can tell the difference between Cmaj and Am by what chord it resolves to. Usually the first and last chord dictates the key of the song - not always though.
#4
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Em is G major

So would a song like, I dunno, Voodoo Chile be in the key of G or E?
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Of course there's such a thing as key of Em.

You can tell what key it is by the flats/sharps that are being used.

For example, C major and its relative minor (Am) have no sharps or flats. You can tell the difference between Cmaj and Am by what chord it resolves to. Usually the first and last chord dictates the key of the song - not always though.

Ok, thanks for that, but can you tell by looking at a tab?
And what if it starts on a different chord to the root chord? How can you tell?
#5
looking at the tab shows you the notes being played. if its a riff, then you'll be able to tell what scale is being used, so you'll know what key it's in.
if you're looking at a chord chart, look at the chords that are in it and play through it, see what chord sounds like it should start and finish on.
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#6
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Ok, thanks for that, but can you tell by looking at a tab?
And what if it starts on a different chord to the root chord? How can you tell?

Some chord progressions don't start off with the root, yes. You can tell the key by what it resolves to though. It's very easy to tell what chord is the resolution too. Pretty much an inherent ability in everyone. When it sounds right, like the progression can be ended at that chord, then that's the chord it resolves around. Could be written as something like: II - IV - I - IV - I or something

As for looking at tabs, once again, just figure out what sharps/flats are being used in the section and you can figure it out.
#8
To make things a little easier, I suggest you learn about the Circle of 5ths.