Originally Posted by: fanbladeThe relative minor of C major is Am. This is the sixth degree of the C major scale. Why is the sixth degree or the Aeolian mode used as the relative minor? Why not say D dorian is the relative minor ?

The 6th scale degree of the major scale is used to start it's relative minor because that is the only mode that will result in the correct set of intervals forming the natural minor scale (aeolian mode). All the optional notes will be minor.

In diatonic scales, the 3rd, 6th & 7th degrees can be either major or minor. The 1st, 2nd, 4th & 5th are supposed to be perfect intervals in major (ionian) & minor (aeolian). The other modes have a different mixture of major & minor intervals as well as flat 2nds, flat 5ths (& a sharp 4th in the case of lydian).

If you start on the 2nd scale degree you get dorian. However, the interval formula for dorian is:

1st - 2nd - minor 3rd - 4th - 5th - major 6th - minor 7th

If you start on the 3rd scale degree you get phrygian. However, the interval formula for phrygian is:

1st - flat 2nd - minor 3rd - 4th - 5th - minor 6th - minor 7th

If you start on the 7th scale degree you get locrian. However, the interval formula for locrian is:

1st - flat 2nd - minor 3rd - 4th - flat 5th - minor 6th - minor 7th

Only by starting on the 6th degree of the major scale do you get the natural minor scale that is the aeolian mode:

1st - 2nd - minor 3rd - 4th - 5th - minor 6th - minor 7th

Lydian & mixolydian, of course, don't work immediately because they have major 3rds. Make sense?

These tutorials are all about the modes of the major scale:

http://www.guitartricks.com/tutorial.php?input=370
http://www.guitartricks.com/tutorial.php?input=770

Hope this helps. Have fun!