Triads consist of three note sequences of diatonic tones from a given scale. Triads can start on any note, but they always include the 3rd and 5th in relation to the root note we start on. Based on the relation of the 3rd and 5th to the root we can arrive at four main qualities of triads:
- Major (M)
- minor (m)
- Augmented (+)
- diminished (°)
How Triads are Formed
A major triad has an interval of a Major 3rd between the root and third and a minor 3rd between the third and 5th. Conversely, a minor triad has an interval of a minor 3rd between the minor triad and a Major 3rd between the third and fifth. The interval between the root and the fifth is a perfect fifth in both instances.
Augmented triads stack two Major 3rds on top of each other and diminished triads stack two minor triads on top of each other. The fifth is always a fifth away from the root but we use accidentals to change their relationship to each other.
Visual Representation of Triads
Triad Crash Course
Notating Triads
Triads are notated in a few ways. The Roman numeral system allows ease of transposition because it does not tie you to a specific triad, but highlights the quality of that triad and its relation to the other triads. There are seven diatonic tones in a scale so we use Roman numerals from one to seven to represent these tones. An upper case numeral represents a Major triad and and lower case numeral represents a minor triad. We use the + & ° to show that our triads are Augmented and diminished.
N.B. We will usually raise the 7th scale degree in a minor scale so that we have the leading tone present. This allows the V to be major and the vii to be diminished as in the Major scale.
Major numerals – I II III IV V VI VII
minor numerals – i ii iii iv v vi vii
A major scale is notated as this: I ii iii IV V vi vii°
A Harmonic minor scale is notated as this: i ii° III iv V VI vii°
Chord Functions
Chords serve various functions. They can be Tonic, Predominant, or Dominant.
A typical pattern for a chord progression would be: Tonic – Predominant – Dominant – Tonic.
The tonic triad, I, can be interchanged with the vi trad because they share two notes in common. The other tone in the vi triad is one tone higher than the tone it resembles in the I triad.
The dominant triad, V, can be interchanged with the vii° triad, because they also share two notes in common and the other note in the sequence also differs by a whole tone. When you use a V7 the vii° becomes even more like the V because three tones are shared between the triads in this scenario.
The predominant functions, ii and IV, always sound like they want to move to the dominant chords, hence they typically appear before the V and vii°.