Reference
Triads
Triads are chords formed of three notes. Triads may be major, minor, augmented, or diminished. The following examples show the structure of each of these chords:
Minor triad: minor 3rd., perfect 5th.
Diminished triad: minor 3rd., diminished 5th.
Augmented triad: major 3rd., augmented 5th.
Major and minor triads are considered to be perfect because they always have a perfect fifth. Diminished or augmented triads are called by the name of the fifth they contain.