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Broadly speaking, major chords sound bright and happy. Minor chords sound dark and sad. The C major scale consists of C-D-E-F-G-A-B-C. The A-minor scale consists of A-B-C-D-E-F-G-A. Those are the same notes, but if you play each of those patterns on a keyboard, the first one sounds happy, the second one sounds sad. A C-major chord (technically, triad) is made up of C-E-G, the first, third, and fifth notes of the scale. Again, sounds happy. The A-minor triad is A-C-E ... sad. But if you play a melody -- that is, one note at at a time -- it isn't always clear whether it's happy or sad. In most western music, though (including virtually all pre-1900 classical music and the vast majority of modern pop), the piece will end ("resolve") with a clearer "happy" or "sad" type of chord. That final chord is what determines the key. (In a huge amount of classical and popular music, the final chord is the same as the opening chord, but not always. When they're different, the final chord tells you the key.) |
That makes it sound like a definitive rule, but it's just a common convention.