Between them, the four kinds of triads mentioned in this chapter—diminished, minor, major, and augmented—account for all triad types available in the major and minor modes. Mathematically, the four types can be explained by the fact that a triad can be composed of two thirds. Because there are two kinds of third—major and minor—this means there are thus four types of triads (see Figure 17.7).
A useful rule of thumb to work out any of these four triads from a given note is the semitone rule:
Diminished Triad: 3 plus 3 semitones
Minor Triad: 3 plus 4 semitones
Major Triad: 4 plus ...