This article in in answer to a reader who asked why some chords in a key are major chords while others are minor, "I noticed a gap in my theory knowledge that you might be able to help with. In playing chords in the Key of C - is there a reason the other chords are minor/dim? In the sense, why is it that the chord progressions (in scale order goes) I - ii - iii - IV - V - vi - viii ? The answer might be "just cause it's the way it is" but perhaps you can shed some more light on the matter. And as a final question, are all major keys chord changes in that order? Would the key of A have the same I - ii - iii ... etc?"
The last part of this question is easy to answer. Major, minor and diminished chords occur in that order in every major key. The chords on the root, fourth and fifth degrees are major, on the second, third and sixth degrees minor, and on the seventh degree diminished.
It's a little harder to explain why some chords are major and others minor, but let me try. Let's use your guitar's B string (2nd from the bottom) to illustrate this, the notes of the C major scale are shown along this string for one octave - from the first to the thirteenth fret below:
Now let's use this scale to build a few chords. A basic chord is made up of the root, third and fifth notes of the scale so we'll pick these notes from the C scale shown above.
First the I chord, C major, start from c at the first fret and count up two notes to the third, e. Notice that the distance between these notes is four frets (or two whole tones) which makes a major third. Now we count up two more notes to get the fifth, g at the eighth fret and we've got a C major chord on the I degree.
Now let's do the same for the ii chord. We start from the root D note at fret three and count two notes up the scale for the third. This time we arrive on F, notice that the distance between the root and this third is only three frets (or one and a half tones) - it's a minor third which gives the chord its minor quality.
Two notes further up and we have the A, fifth note at the tenth fret and we have a D minor chord on the ii degree. Notice the distance between the root and the fifth is seven frets for both the C and D chords.
If you perfom the same exercise for the other notes in the C scale you will get chords with three fret minor thirds on degrees ii, iii, vi and vii. On vii you will find that there is also one less fret for the fifth, so we get a diminished chord.
Try this exercise with some other scales and you'll see for yourself that the pattern repeats in the same way for all of them. Chords on some scale degrees have a major third interval, while chords on some others have minor thirds. The major and minor third intervals occur always on the same degrees of the scale leading to the familiar pattern of major chords on the first, fourth and fifth degrees and minor chords on the second, third, and sixth degrees.
Now you know where the major key formula of major and minor chords comes from and you can find the major, minor and diminished chords in any major key.
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