Types of Triads

General Definition:

Triads are, as the name implies, collections of notes that include three different pitch classes.

Tertian Sonorities:

In tonal music, the term triad is often limited to collections of notes a third apart from each other.
In this use the collection C - E - G is a triad, while the collection C - E F is not.
Note that C - E - - A is a triad because these notes can be rearranged in thirds as A - C - E.

Diatonic Triads:

The triads in common use in tonal music are called diatonic, meaning that they can be derived from the diatonic (major or minor) scales.
These triads are divided into four types according to their arrangement of major and minor thirds. See and hear the examples below.

Major triad	Minor triad
Diminished triad	Augmented triad

Triads in practice

It's important to realize that the examples above are merely the simplest ways to show how triads are formed. In practice, and in your own composition, you can come across many different ways to use triads. Here are just a few, using one minor triad.

Harmonic, root position, close spacing.	Harmonic, second inversion, open spacing.
Harmonic, first inversion, open spacing.	Melodic, root position. (Arpeggiated).	Melodic, root position. (Arpeggiated).