Zero,

What Kevin says about lowercase for chords that use a minor 3rd up from the root and uppercase to denote chords that use a major 3rd above the root is an excellent way to distinguish the two.

This is something that developed in the last 100 years or so. Prior to that the classical musicians would use VI for the default, diatonic chord based on the 6th note of scale of the key signature. Thus in the key of C major, VI classically means Am. (Even today, there are theorists who prefer not to use lowercase Roman numerals.)

When chords outside diatonic harmony were used in the past, there were different techniques to employ. In the case of A7, this would have been considered a secondary dominant 7th and so it would have been written as V7/II (since it is the dominant 7th chord of II (Dm) if note II were temporarily considered the key at that point in time - that is V7/II leads to chord II).

Thus the progression I VI7 II7 V7 classically means...
C Am7 Dm7 G7

By comparison, the progression I V7/II II7 V7 means...
C A7 Dm7 G7

Here's an article about secondary dominants in Wikipedia.

http://en.wikipedia.org/wiki/Secondary_dominant

Regards,
Noel

MY SONGS...
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