|The Circle of 5ths|
Out of the potentially continuous infinity of pitch available from slide whistles and fretless strings, Western practice has fixed upon a finite set of discreet values as the set of notes available for crafting musical expression. They are known as semi-tones, of which there are exactly 12. Because of the basic phenomenon of the octave where a note value recurs at a higher or lower pitch with the same fundamental musical identity (e.g. high C, middle C, low C, etc.), the full spectrum of pitch from low to high is reducible to these 12 semi-tones. The circle of fifths is divided into 12 sections, each one rooted on one of these 12 tones.
Out of a complete universe of 12 semi-tones, Western music generally builds most of its melodies and harmonies from a working subset of 7 notes at any one time. (Remember, the 8th note is the octave, a repeat, a return to the beginning again). Such a subset is known as a key or a scale. Out of 12 semi-tones, there are many possible subsets of 7. But Western music has largely concentrated on only 12 of these, a sequence of 7 tones rooted at each of the 12 semi-tones yielding exactly 12 major scales. These are given symbolic names and are shown as labels around the outer circle of 5ths using the letters A through G and symbols for sharp and flat.
In addition to the 12 major scales, Western music uses 12 additional scales which are named "minor". But it turns out that the minor scales are just the major scales starting at a different root. They are have the same selection of notes as one of the major keys but they use a different ordering. Well, the same ordering, but starting from a different note, a root note that has a different character than its major counterpart. In summary, this doubles the set of available keys to 24, with two different scales rooted on each of the 12 semi-tones, one major and one minor. For each major key labeled on the outer circle of 5ths, there is a cooresponding minor key labeled on the inner circle of 5ths. Two keys or scales share the same wedge of the circle: the major and its relative minor, the minor and its relative major. For example, A minor is closely related to C major.
The general tonal universe of Western music comprises 24 keys, 12 major and 12 minor, built on this set of 12 semi-tones. In order to create a comprehensive catalog of music compositions, Bach chose, in his Well-Tempered Clavier, to create one piece in each of these keys, hence, the 24 preludes and fugues of Book 1. Bach published a second set, known as Book 2 of the Well-Tempered Clavier, itself, another complete universe of pieces, one for each of the 24 available keys. And so did Chopin compose 24 preludes, Shostakovich, his own set of 24 preludes and fugues.
Besides encompassing the full set of available keys, the circle of 5ths also governs (or describes) the way an individual composition moves. Within each scale, rooted at a particular semi-tone, the 5th note has a powerful role to play. Taking the key of C major as an example, its 7 tones are named C, D, E, F, G, A and B. The fifth tone is named G. G is the fifth tone in the key of C. Starting from any key in the circle (major or minor, outer or inner), you will see that the next key moving in a clockwise direction is in fact the 5th. So C moves clockwise to G, A minor to E, and so on. This circular movement has everything to do with the underlying dynamic of musical movement, the coherence of melody and harmony within a given key. It also turns out that a counterclockwise movement has a similar significance. As movement from C to G has a crucial musical meaning, so does the opposite movement from C to F (from A minor to D).
While most compositions of Western classic music are firmly based in a single ruling key (exactly one of the nodes on the circle of 5ths) most compositions shift to one or more alternate keys for contrast within a single dramatic narrative. Thus, a musical selection in the key of C major will migrate temporarily into another key such as G major. Or perhaps A minor. This is known as modulation. Curiously, the choice of a new key as the target of modulation is also well represented by the circle of 5ths. Most instances of modulation occur in simple movements around the circle of 5ths, often a single step clockwise, counterclockwise or, a shift to a relative key within the same wedge, moving from the outer circle to the inner, or, from the inner to the outer (e.g. C to A minor or A minor to C).
This is a brief, high-level summary. It is both logical and somehow mystical. It is a beginning of understanding, as well as a contemplation to return to again and again. The circle of 5ths encapsulates the full universe of Western tonality and provides a map for how music moves both within a key and how it modulates to different, related keys. It is a core unifying force, recurrent at many levels.
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