The cycle of 5ths developed as the major/minor tonality became established in the work of composers at the beginning of the 18th century, which was the period of the late Baroque. The order of tonality had become a feature of western classical music, an order which was based on the relationship between a tonal centre (the tonic triad in major and minor keys) and secondary chords. Therefore harmonic sequence drove towards a cadence and tension created as a result of moving away from the tonic. The cycle of 5ths is a means of moving the harmony away from and back to the tonic through descending movement (by a 5th).
Fig. 1
When adding an accidental to the dominant chord (to the leading noted – G#) it becomes a major chord. Remember that it’s the major dominant chord which is usually used in V – i cadences.
The sequence of chords ii – V – i is very common especially when approaching a cadence. The supertonic chord can be either a diminished chord (B-D-F♮, in A minor) or a minor chord (B-D-F#) if supporting a melody that uses the melodic form of the minor scale – see figure 2 below.
A cycle of 5ths is a device that you can use to extend melodic ideas.
Fig. 2
As a rule, the melodic form of the minor scale is used in ascending melodies which are harmonised with chords ii or iv. Using the harmonic scale would involve a leap of an augmented 2nd between the sub-mediant and the leading note in the melody. Experiment by playing a F♮ rather than F# in example 2a.
The device enables you to extend melodic material by moving far from the tonic chord. Consider how Corelli does this in the excerpt below. He uses a diatonic cycle which means that he goes through a cycle of chords as they appear in the relative major key (F major).
In contrast, the excerpt from Schubert's work converts the largest tonic to the smallest tonic (E♭) before going through a cycle of 5ths where the bass drops a perfect 5th each time. Click on these links for examples.
How to decide on the notes of the melody.