How Chords Work

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A triad is a set of 3 notes with a certain interval between each note. It can also be thought of as 2 intervals stacked on top of each other. For a major triad you stack a major third then a minor 3rd. For a C major triad this would be C-E-G. (C-E is a major third, E-G is minor third). Or more commonly, you can think of it as 2 intervals from the root which would be a major third and perfect fifth (C-E is major third, C-G is perfect fifth).

There are major, minor, diminished and augmented triads.

The 3 notes of the triad can be played together which is called a chord, or played individually which is called an arpeggio.

If we build triads on each note of the major scale we get the following sequence which is true for all major scales: major triad, minor triad, minor triad, major triad, major triad, minor triad, diminished triad.

A major chord is shown with a capital letter only and implies a major chord. So C major is represented by C. A minor chord is shown with a capital letter and a lower case m or sometimes a minus sign. So D minor is represented as Dm (or D-). A diminished chord is shown as a letter and the word ‘dim’. An augmented chord is shown as a letter and the word ‘aug’ or ‘+’. These triads are usually shown in roman numeral form, making them generic and applicable to all major scales:
I ii iii IV V vi viio (the upper case numerals are major, lower case are minor)

In the key of C this gives us C Dm Em F G Am Bdim. The pattern of major, minor and diminished triads remains identical for any major scale. This pattern is major, minor, minor, major, major, minor, diminished