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Take a C major scale, and pick each other note, starting from each note

        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?

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I play several songs in C and have written one that use mainly C, G, D, and Em. Nothing about Em sounds wrong to me in that context, and E doesn't fit better in those songs at all. I can't speak for classical pieces, though. – Matthew Read Jul 4 '11 at 21:02
Can you give the musical example? You mentioned a specific melody. In common practice, a Major III is likely a temporary tonicization of a different key. – NReilingh Jul 4 '11 at 23:47
sometimes I do feel the same. it's like the little brother of the B chords (which I rarely see it's being used in popular music).. but Em is still sounds good in most of the time. – Phelios Jul 5 '11 at 5:49
I'm with @Matthew on this - the iii sounds(to me) far more appropriate/normal than the III – Dr Mayhem Jul 5 '11 at 10:32
In the "Common Practice Period", when major keys are concerned, iii generally leads into ii6, IV, V, or vi, and is lead into itself by either I or vi. In minor keys of the period, this is when iii is replaced with III, which leads similarly, except not to V, and to VI instead of vi. III is also commonly led into by not only i and VI, but also VII. – UtopiaLtd Jul 5 '11 at 23:39

Yes, there is a very good explanation. The III chord can be much better expressed as a secondary dominant. Basically, III happens to be the V of vi.

A secondary dominant is using a non-diatonic chord (in other words, a chord that does not naturally belong in the scale, containing an accidental, like the G#) to provide a means to change key or shift to another tonality.

The III chord, or V/vi as it probably should be written, provides an interesting way to shift the the harmonic minor of vi. For example, in the key of C. You could play an E major leading to an A minor. This often works especially well when the melody goes G - G# - A:

melody:  G------- G#------ A-------
harmony: C------- E------- Am------

Of course, there's nothing "wrong" with using the normal iii chord within it's key. It has been traditionally abandoned because there's not a particularly good use for it. It could theoretically sub for the I chord, but the much better choice for a substitution is vi.

Hope this helps!

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