Three chords are considered to perform primary functions. The triads built on the tonic, subdominant, and dominant are often referred to as the primary triads because of their strong relationship to each other. These are the tonic, the chord built on the first degree of the scale, the dominant built on the fifth degree of the scale, and the subdominant built on the fourth degree of the scale. Notice that between them, these three chords define all seven notes of the scale. This is an important observation for the simple reason that by using these three chords, it is therefore possible to put a harmony to every note of the scale. This means that, feasibly, every melody could be harmonized using these three chords alone.

As a group, the tonic, subdominant, and dominant triads are called the three primary chords. Lying between the dominant and subdominant, the tonic chord represents the seat of tonality, while the dominant chord—whose root lies a perfect fifth above the tonic—acts as the principal supporter of the tonic. The subdominant—whose root lies a fifth below the root of the tonic chord—similarly performs a supporting role to the tonic. In this way, both dominant and subdominant help to stabilize and support the tonic chord. This means that the function of the primary chords as a group is to preserve and maintain a stable sense of tonality throughout the chord progression.

The tonic and dominant triads of a key have a very special relationship. This relationship has to do with their particular functions in maintaining a sense of tonality.

• The tonic chord represents the seat of tonality.
• The dominant chord represents the driver of tonality.

All other chords perform a secondary function within the key, which means that they are chords that usually act as intermediaries between the three primary chords or are used as temporary substitutes for them. This, in turn, gives chord progressions the possibility of greater interest, diversity, and the expression of a wide range of emotions depending upon the particular chords chosen for use in the chord progression.

The mediant is the chord whose root lies halfway between the tonic and the dominant. The submediant is similarly the chord whose root lies halfway between the tonic and the subdominant. The supertonic, as its name suggests, is the chord whose root lies one step above the tonic. Finally, the leading tone triad is the chord whose root lies a semitone below the tonic.