Originally posted by StratsGalore
What's the difference between a 7th chord and a minor 7th chord?

Here's the simple answer to this question - dominant 7th chords are built off the 5th (V - dominant) tone of the major scale. For D7, go to the G major scale, and count up the 1st, 3rd, 5th, and 7ths tones from D. If you've done it right, you should get D F# A C. Minor 7th chords are built off the second tone (II - supertonic) of the major scale. For a Dm7, go to the C major scale and count up 1,3,5,7 from D. You'll get D F A C.

Basically, the difference is which scale tone you start on. The second tone of the major is scale is dorian (minor), so you get minor triads when you count up. The fifth tone is mixolydian (major), so you get major triads.

Ok . . . maybe that wasn't so simple. But it's the way I think about it from piano. In music theory, there's a lot of different ways to visualize the same thing. You just have to find the way that works for you.