How is the key of a chord progression determined?

Question

For instance use:

Em C G D

; what key and why?

Answer 1

Your chord progression is incomplete unless the ending note of D becomes the new key signature. Otherwise, Em would be the key.

But continuing the progression with Bm, F#, G, A7, and back to D makes a good transition to keep the key signature in D major.

Another approach is to continue from your progression into B7, Em, Am, D7, and then to G as the new key.

Answer 2

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If you start it on the Bm7, then it's in B minor. If you start it on Gmaj7, it's in G major. If you start on the Amaj7, it's in A major. There's a reason you started it with the Bm7 chord. It sounds good and seems to make logical tonal sense with the chords in that order. In that case, you're in B minor. Just putting together a couple of chords doesn't necessarily determine a key. Your chords have to be in some sort of arrangement with a beginning and an end. That's the only way to know what key you're in. General rule of thumb: Whatever chord you start and end with is the key of your song. This obviously isn't always true, but it's a good place to start.

Answer 3

3 reasons why it's probably Em.

1st. from playing it it sounds like it's probably the key of Em, which is the relative minor of G, (same key as G - 1 sharp).

2nd, is because all the chords fit into the key of G/Em, but not another key. If it were the key of C (no sharps or flats) you couldn't have a D chord, if it were in the key of D (two sharps) you couldn't have a C chord.

3rd reason, it starts on Em. But here's why major keys and their relative minors are tricky. Play the progression and end on an Em right after the D, like this: Em C G D Em. Now do it and end on a G, like this: Em C G D G. They both fit don't they?

Still, It's probably Em, and this is why:

It sounds like Em,
It contains chords that are consistent with Em,
It starts on an Em, and resolves back to it.

Answer 4

Em,C,G,D is in the key of G,,

The last two chord is in a half cadence. Which means that those four chords are one musical phrase.