Knowing Which Key You Are In
Question by Max
If I play a progression such as A minor - E minor - C major, am I in the key of A minor or C major?
Key is defined by the chord to which your progression resolves, known as the tonic chord. This is like the feeling of returning home.
Since C major and A minor are relative major/minor keys, you can resolve to either quite naturally from a few of their related chords.
But key can be quite subjective, especially when you have two very closely related tonics like Am and Cmaj.
C major resolves most strongly from F major or G major (the 4 and 5 chords).
However, the resolution to Cmaj isn't as strong from a chord like Em (the 3 chord).
Because Em is the natural 5 chord of the A minor key, there is more "pull" to Am than Cmaj as a tonic chord.
But like I said, this is all quite subjective and you really have to let your ears be the judge. Some keys are fairly ambiguous, some are more obvious.
Think of key as being "implied" rather than set in stone.
A lot of this implication is done with the opening chord setting the scene/mood. If you start with Am and move through to Em, chances are you'll feel more of a pull to Am as that "home" chord.
However, start on Cmaj, and Em will feel more of an intermediary chord. It's all about context.
Keep playing around with different chord combinations and explore this "pull" and resolution effect. I go into this in some depth in the guitar chord progressions section.