Take a C major scale, and pick each other note, starting from each note
C-D-EF-G-A-BC-D-EF I: C E G = C ii: D F A = Dm iii: E G B = Em IV: F A C = F V: G B D = G vi: A C E = Am vii0: B D F = Bdim
This is what they call Roman numeral system -- very nice intellectual construction.
My question question is about
iii=Em chord -- it really sounds wrong, when you play it with those surrounding chords. The
III=E fits there much better. And it seems that I'm not alone with such a judgment -- it is a common practice in many classical pieces to play
III "instead of"
iii. And also the "wrong" note G# (in this key) is used in the melody.
There must be some "intellectual explanation" for it. Am I right?