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> C_major D_minor E_minor F_major G_major A_minor B_diminished C_major

A good way to see where this comes from is to note that the major scale with C as the tonic ("root") note is

    C D E F G A B
(the white keys on a piano), and starting at any note in the sequence and taking every other note (for a total of three) gives you three-note chords called "triads":

    C    = C E G (skip D, take E, skip F, take G)
    Dm   = D F A
    Em   = E G B
    F    = F A C
    G    = G B D
    Am   = A C E
    Bdim = B D F
The intervals between successive notes are (major or minor) thirds, hence these chords are constructed by ascending (moving up the scale) in thirds. The Wikipedia page does a good job of explaining how you can construct chords like this:


You can take four notes at a time instead of three to get seventh chords.

I've been playing for quite some time, and have also known bits and pieces of theory for about the same amount of time, but I've never really applied the latter to my playing until recently. I'm getting into playing Bach and so on and find that I'd really like to understand, e.g. Roman numeral analyses, and being fluent in basic theory and how it "looks" on your instrument is key to this.

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