As I previously stated, the circle of fifths is essentially a study in Dominant-Tonic relationships, with each note of the circle resolving to its nearest counter-clockwise neighbor. With this logic, we can assume that any note functioning as a Tonic is also capable of functioning as the Dominant to its nearest counter-clockwise neighbor, the Subdominant.
The circle of fifths, rotated slightly, showing the movement of Tonic (I), Subdominant (IV), and Dominant (V) chord/degree/key relationships. The tendency is to move toward and away from the Tonic in a counter-clockwise motion.
The terms Tonic-Dominant-Subdominant can be used to describe modulation between tonal centers, to identify the degrees of a scale, and to name chords in a progression. There’s really no place that these three words are not applicable in the entirety of western music, in everything from Bach to the Blues.
The Tonic, Dominant, and Subdominant chords as they appear on the grand staff. Each revolve around another through shared notes. The sum of all the notes combined can be octave-reduced to create a major scale; F major in this example.
The Subdominant is surrounded by a bit of ambiguity, and maybe even a bit of controversy. By ascending clockwise around the circle of fifths from any pitch, six successive notes can be octave-reduced to form a major mode with the exclusion of only one missing note: the Subdominant. The Subdominant resides below the tonic and does not come from the same overtones/partials as the other pitches produced in a major scale. Because of this, the Subdominant has a tendency to exist through a change in key rather than as a chord that belongs to the Tonic. The I chord can be thought of as a chord that is shared with the subdominant key rather than the IV chord being a part of the tonic key.
The F major scale is spelled out here as it appears around the circle of fifths.
From Mendelssohn’s Songs without Words, Op. 85, No. 1; measures 39-41. This excerpt illustrates the movement of the Tonic as it briefly modulates to the Subdominant key of B-flat. The I7 is acting as the Dominant of the IV chord. The V7 resolves back to the home-key of F major.
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