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The most important harmonic sequence is called the descending fifth progression.
The descending fifth progression is a non-modulating sequence which is based on the diatonic transposition by four scale degrees down. We can also say that we are applying the interval of the descending fifth (hence the name descending fifth progression). While a non-modulating sequence can diatonically transpose a chord progression of any length, in the case of the descending fifth progression, the length of the chord progression is one chord. That is, the root of the next chord is obtained from the root of the previous one by going four scale degrees down.
The complete descending fifth progression begins and ends with the same scale degree (most often, this is the tonic), visiting other scale degrees once. Thus, the scale degrees of the complete descending fifth progression which begins on the tonic are: the first, fourth, seventh, third, sixth, second, fifth and first. The next figure illustrates this for the C major key and the A minor key.
Figure: The scale degrees of the complete descending fifth progression, beginning with the tonic, in the C major and A minor key
The complete descending fifth progression does not occur often in compositions. More often, only a part of it occurs. For instance, the chord progressions V – I and ii – V – I occur very often in major keys. Frequently, they appear after the tonic chord. That is, I – V – I or I – ii – V – I in major keys. However, when the complete descending fifth progression occurs in a composition, it is quite noticeable and interesting. Often, the descending fifth progression is realized using only diatonic triad chords, as in the following two examples.
The next figure shows a charming example of the complete descending fifth progression from Mozart's Piano Sonata in C major K. 545. It uses only diatonic triad chords.
Figure: The complete descending fifth progression in Mozart's Piano Sonata in C major K. 545
The next figure shows a remarkable example of the double descending fifth progression starting on the fourth scale degree. We see that it visits each scale degree twice (except the fourth scale degree which occurs at the start, end and in the middle). The example is taken from Bach's Prelude No. 15 in G major from Well-Tempered Clavier, book I. It uses only diatonic triad chords.
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