A triad is a chord of three notes. For example, C – E – G is the C major triad. C is the root note, E is the 3rd note of the scale, and G is the 5th.
The two main kinds of triad are called major and minor. The major triad has a major 3rd (eg C – E – G) and the minor triad has a minor 3rd (eg C – E flat – G).
Dominant and subdominant
In any major key, the chords built on the first, fourth and fifth degrees of the scale are all major. In the key of C, these are the chords of C, F and G. They are also sometimes called I, IV…