為甚麼Minor Key會有三個Scales, 而Major Key 只有一個呢？

1)Minor passages are sections of a piece of music that is in minor key. For instance, Beethoven 5th Symphony c-minor, the first theme is in minor mode, which is so called minor passages.
2)In the theory of harmony, only a major form of dominant can arrive into the major or minor tonic. The middle note of the dominant chord is the seventh note of the minor scale; therefore, it must be a major chord when using harmonic scale. ONLY in old Chinese pop songs will use the minor form of dominant, however, this example happens only in minor keys. For instance, try finding the different feelings by playing an E-minor chord, then an A-minor, and by playing E-major- then A-minor. I believe you will find the E-major to A-minor is more resolved.
3)Secondary Dominants refer to each of the Dominant chords of the chords originally constructed from the original key. In the example I gave, if you have Piano, try play C-major, A-minor, F-major, D-major, G-major, and C-major chord in a sequence, you will find D major go in to G major very fluently because D major is the dominant of G major, then, since G-major chord is in the key of C major, then D major is a Secondary Dominant of C major. E-major chord is the Dominant of A-minor, and D-major is the Secondary Dominant of C-major.