Let's now take this principle of tertian harmony and apply it to the major scale to generate the diatonic triads. As you look at chart number 1, place 3 fingers on every other note in the key of C starting on the 1. The resulting notes will be C, E, G. These are the 3 notes in the I chord in the key of C and they form a C major triad. If you take your three fingers and move each note up to the next note in the scale, the resulting notes will be D, F, A. This is the spelling of the ii chord in the key of C, which is a Dm.
If you continue with this approach, you'll generate 7 diatonic triads which will always follow the chord qualities which are listed in the lower left-hand box of this chart. Memorize these chord qualities with their respective positions: I, IV, and V are major triads, ii, iii, and vi are minor triads, and the vii chord is a diminished triad. The definition of a triad is a 3 note chord built in intervals of thirds which you now know are simply every other note of the scale. Notice how any two triads that are a 3rd apart will share two common tones.