Originally Posted by: KasperowThanks for the explanation :)[/quote]
Right on! Great job, Seb. :)

I cover the basics of music theory & Roman Numeral analysis in this tutorial.

www.guitartricks.com/tutorial.php?input=495

The quick answers:

1. Upper case Roman Numerals indicate major chords.

2. Lower case Roman Numerals indicate minor chords.

3. The structure/order/system of major, minor & diminished chords in a major (and minor) scale is due to building chords on the notes of the scale using only notes of the scale according to Triadic Harmony. See the music theory tutorial for details.

4. You can, however, play any chord you want anytime you want regardless of the scale. And the system of Roman Numerals can be used to reflect that also.

In fact, the vast majority of music has accidentals & modulations. The major & minor scales are default settings, they are frames of reference. From Bach & Beethoven, to Broadway, Blues & The Beatles, stepping in & out of the scales, the established key, including chords not in the key, is how music is made interesting.

The scales are not "lines you have to stay inside of", they are not "boring rules". They do form the basic alphabet, the raw materials of music. Building them into harmonies & chords forms the basic grammar of music. It's up to use you figure out how to use these raw materials to tell a story. :)
[QUOTE=Kasperow]One quick question, though: How can you know the vii Chord of the Major Key and the ii Chord of the Minor Key are diminished Chords? They're written the same way as a Minor Chord. Is this some kind of rule of thumb, or is there an explanation behind it?

They are diminished because if you build a triadic chord on that note of the scale using only notes from the scale, then that is the resultant chord.

Let's build a triadic chord on the 7th note of the C major scale.

The 7th note is B. Using the rules of triadic harmony you start at B. Go up 3 scale degrees to D, go up three more scale degrees to F.

That is your chord built on the 7th note of the C major scale:

B-D-F

The intervals (the distances) between those notes forms a diminished chord:

B (root)
D (up a minor 3rd from the root)
F (up a flat 5th from the root)

B (1st)
D (min 3rd)
F (flat 5th)

B Diminished chord.

Let's build a triadic chord on the 7th note of the C major scale.

The 7th note is B. Using the rules of triadic harmony you start at B. Go up 3 scale degrees to D, go up three more scale degrees to F.

That is your chord built on the 7th note of the C major scale:

B-D-F

The intervals (the distances) between those notes forms a diminished chord:

B (root)
D (up a minor 3rd from the root)
F (up a flat 5th from the root)

B (1st)
D (min 3rd)
F (flat 5th)

B Diminished chord.

Let's build a triadic chord on the 7th note of the C major scale.

The 7th note is B. Using the rules of triadic harmony you start at B. Go up 3 scale degrees to D, go up three more scale degrees to F.

That is your chord built on the 7th note of the C major scale:

B-D-F

The intervals (the distances) between those notes forms a diminished chord:

B (root)
D (up a minor 3rd from the root)
F (up a flat 5th from the root)

B (1st)
D (min 3rd)
F (flat 5th)

B Diminished chord.

Let's build a triadic chord on the 2nd note of the A minor scale.

The 2nd note is B. Using the rules of triadic harmony you start at B. Go up 3 scale degrees to D, go up three more scale degrees to F.

That is your chord built on the 2nd note of the A minor scale:

B-D-F

The intervals (the distances) between those notes forms a diminished chord:

B (root)
D (up a minor 3rd from the root)
F (up a flat 5th from the root)

B (1st)
D (min 3rd)
F (flat 5th)

B Diminished chord.

The reason we find a B dim chord on both is because C major & A Minor are relative scales.

C major is relative major to A minor.
A minor is relative minor C major.

Notice that the same order occurs once you offset the scales properly.

Notes: c - d - e - f - g - a - b
Major scale: I - ii - iii - IV - V - vi - vii dim
Minor scale: III - iv - v - VI - VII - i - ii dim

Major - minor - minor - major - major - minor - dim

The diminished chord has a minor 3rd, so we use lower case Roman Numeral: vii. In typical notation we use a little superscript circle next to the chord numeral to indicate. In text format I usually just indicate this with "dim":

B diminished in C major is: vii dim.

You might also find these tutorials of interest on this topic.

Circle of Fifths
www.guitartricks.com/tutorial.php?input=835

Intro To Improv
www.guitartricks.com/tutorial.php?input=876
www.guitartricks.com/tutorial.php?input=483
www.guitartricks.com/tutorial.php?input=491

And not necessarily for playing technique, but to see & hear how these diminished chords, modulations & so forth actually get used.

http://www.guitartricks.com/tutorial.php?input=973
http://www.guitartricks.com/tutorial.php?input=1166
http://www.guitartricks.com/tutorial.php?input=1205

Have fun with all that!