(This post assumes some knowledge of music theory. If you need, you can refer to the Wikipedia articles that I have linked, and especially the article on music intervals.)
In traditional music theory books, pentatonic scales (5-note scales) are generally mentioned briefly and as a means to express a folk or exotic sound, in contrast to the standard sound expressed by the classical diatonic 7-note scales. The view presented in these manuals is often “key-centered”: tonality, that is to say the classical major and minor keys, holds a central position, while other more or less common sets of pitches (scales or chords) play secondary roles, revolving around tonality and acting as temporary alternatives.
However, at least since the late XIX century, more and more music has been written using non-tonal pitch material. In particular, 7-note modal scales derived from the diatonic major scale and a specific kind of pentatonic scale have been used frequently in different genres and styles.
The pentatonic scale I am referring to is known under two names: “major pentatonic scale” and “minor pentatonic scale”. This can be confusing, but there is a simple reason. As any scale in general, pentatonic scales also have their own modes, and those names just refer to two different modes of the same scale. That is to say, they refer to the same scale but starting on two different notes. As an example, let’s consider an instance of this common pentatonic scale:
C D E G A (C).
The scale’s modes (essentially, its “circular permutations”) are:
*I C D E G A
II D E G A C
III E G A C D
IV G A C D E
*V A C D E G
The asterisks mark the modes that are used to name the scale. It may be called either the C major pentatonic scale or the A minor pentatonic scale. The adjectives “major” and “minor” indicate respectively that a major triad (CEG) is included in the first mode and that a minor triad (ACE) is included in the fifth mode.
The “minor pentatonic” label is more popular, especially because the minor mode (mode V) is the mode of reference for the so-called “blues scale”. (I will come back to it at the end of this article.)
So, the scale is generally called either “X major pentatonic scale”, where X is the root of the first mode, or “Y minor pentatonic scale”, where Y is the root of the fifth mode. To avoid confusion, from now on I will refer to the scale as either the “major pentatonic” or simply the “pentatonic”.
Note the mode’s tone-semitone pattern: T, T, T+S, T, T+S. Between any two adjacent degrees of the major pentatonic scale there can be either a whole tone (T) or a whole tone and a half (T+S). The semitone is not part of the sound of the pentatonic.
Pentatonics can be seen from many different points of view. I will concentrate mainly on one aspect: using the major pentatonic as an alternative to the diatonic major scale or to the modes of it. But first let’s introduce some more useful nomenclature.
We can assign a numeric symbol (or a couple of alternative symbols, to indicate fundamental enharmonic equivalences) to each step of the chromatic scale. These numeric symbols identify the interval that the specific step forms with the scale’s root, which is itself indicated by an R. Thus, relative to a C root, the chromatic scale is:
C, C♯/D♭, D, D♯/E♭, E, F, F♯/G♭, G, G♯/A♭, A, A♯/B♭, B
and the corresponding numeric symbols are:
R, ♯1/♭2, 2, ♯2/♭3, 3, 4, ♯4/♭5, 5, ♯5/♭6, 6, ♯6/♭7, 7
So, between C and C♯/D♭ there is an augmented unison / minor second, etc. (♯ means augmented; ♭ means either minor or diminished; a simple integer means either major or perfect — refer to the Wikipedia page on intervals for more on this nomenclature.)
We can now apply this principle to any diatonic major scale and to any major pentatonic scale. There are 12 diatonic major scales and 12 major pentatonics, one for each note of the chromatic scale. For example, by considering both scales with C as a root, we have:
C D E F G A B (C major scale)
R 2 3 4 5 6 7
C D E G A (C major pentatonic scale)
R 2 3 5 6
One can easily see that the major pentatonic scale is a diatonic major scale without the 4th and the 7th degree. This is a very important fact. Actually, the perfect 4th and the major 7th are often mentioned as the characteristic tones of the diatonic major scale.
But there also exist two more species of 4th and of 7th: the augmented 4th and the minor 7th. Therefore, by adding different species of 4ths and 7ths to a major pentatonic scale, we can “extend” it in four different ways. More precisely, the major pentatonic scale implies not only the diatonic major scale, but also three more scales each including a 4th and a 7th of a different species:
1) R 2 3 5 6 + 4 and 7 = diatonic major scale or Ionian mode
2) R 2 3 5 6 + ♯4 and 7 = Lydian mode
3) R 2 3 5 6 + 4 and ♭7 = Mixolydian mode
4) R 2 3 5 6 + ♯4 and ♭7 = Lydian Dominant mode
The Lydian Dominant mode is an advanced sound (it’s the fourth mode of the melodic minor scale), and I will not consider it further in this article. The other three modes are respectively the first (Ionian), the fourth (Lydian) and the fifth (Mixolydian) of the major scale. Another way to express this fact is that each major pentatonic implies three major keys.
So, for example, the sound of a C major pentatonic scale played over an isolated C major triad is vague and not sufficient to establish a unique key. That is because, from a tonal point of view, the C major triad can be a first degree in C major, a fourth degree in G major, or a fifth degree in F major. However, it’s exactly because of such vagueness that it is possible to extend the pentatonic scale in at least three different ways and turn it into a Ionian mode (4 and 7), a Lydian mode (♯4 and 7), or a Mixolydian mode (4 and ♭7).
This also means that each major scale (and so also each mode derived from it) implies three major pentatonic scales. They are built respectively on the major scale’s first degree, fourth degree and fifth degree. (The three pentatonics fall on different specific degrees of each other mode.)
From another, similar but somehow opposite point of view, we can use a major pentatonic scale to express a modal sound.
For example, let’s take C Lydian, i.e., C D E F♯ G A B. The characteristic note of the Lydian mode is the augmented 4th. In this case, it is F♯. However, F♯ is not included in the major pentatonic built on the root of the mode (i.e., the C major pentatonic: C D E G A). So, this pentatonic is not particularly appropriate to express a Lydian sound.
Fortunately, because each mode implies three major pentatonics, there are two more pentatonics available. One is built on the second degree of the mode (D E F♯ A B) and the other is built on its fifth degree (G A B D E). We can see that F♯ is included in the former, which is the D major pentatonic. So we can effectively play in C Lydian by playing the D major pentatonic. Because this scale includes the characteristic tone of the C Lydian mode, it is sufficient to express this sound. (The two missing tones, C and G, are respectively the root and the perfect fifth of C Lydian. These are very strong scale tones, functionally speaking, and so they “resonate” implicitly even if they are not played.)
The sound of the major pentatonic is simpler, but also stronger, than the sound of the diatonic major scale or of any of its modes. The two minor thirds included in the pentatonic make it possible to move across the sonic space by wide steps, and the absence of semitones opens up the sound. Therefore, when performing, improvising, or composing, one can use expressively a major pentatonic scale instead of a corresponding diatonic major scale. Vice versa, the major pentatonic scale can also be helpful to “navigate” a chord progression, as a first exploration of it, to find more extended scales that might sound well over the progression. Reducing a progression to a pentatonic skeleton is an excellent way to internalize the progression, and pentatonics can be helpful to identify a change of tonal center. (Actually, when moving from a tonal center to another, it’s easier to switch between pentatonic scales than between diatonic 7-note scales.)
The sound of the major pentatonic scale has progressively become more and more important in several genres and styles. Most notably, blues melodies are based on a slightly modified version of the fifth mode of the major pentatonic (which is called the “minor pentatonic”, as I mentioned earlier). North American folk music in general has a very characteristic pentatonic sound, as also the corresponding European folk music from which it mostly derives. Blues and country music have influenced styles such as jazz, rhythm and blues and rock ‘n’ roll, which in turn influenced later developments such as rock, funk, soul, pop, etc. The pentatonic sound is also prominent in Latin music. In general, the genres and styles created mostly by the African slaves and their descendants, in north, central and south America, typically use some kind of pentatonic scales. These are probably a reinterpretation of African pentatonic scales. Pentatonic scales are also very typical of the music of Eastern Asia.
Indeed, there exist several different pentatonic scales. The major pentatonic is just the most common pentatonic in the Western tradition, including Western music genres influenced by African music. The global diffusion of pentatonic scales suggests that different cultures and traditions have generally preferred 5-note scales to 7-note scales.
But what is the reason behind such variety of pentatonic scales? I would like to suggest two hypotheses. The first is that all the different pentatonic scales would derive from a single, primitive and regular pentatonic scale made of equal steps (a “symmetric” scale). The second is that all the different pentatonics would tend at least to resemble such ideal pentatonic scale, which, however, in fact would have never existed. I consider this second hypothesis more likely (of course, there might be a third option).
Dividing the octave into five equal parts seems theoretically simpler than dividing it into five unequal parts. Whether this “perfect” scale has actually existed or was just an ideal in the mind of musicians, it does not seem so unreasonable that the many different pentatonics that we find in different musical traditions evolved as different implementations of such scale.
The specific differences might be due to stylistic preferences or even to practical difficulties when trying to implement the symmetric pentatonic into actual music or on real instruments. (For example, and significantly, the fundamental interval of a perfect fifth is not included in the symmetric pentatonic.)
Interpreting the different real pentatonic scales as imperfect representations of a perfectly symmetric pentatonic scale might also be a key concept to understand the unusual structure of the so-called “blues scale”. This scale is generally presented as a 6-note scale made of a minor pentatonic scale plus an added ♯4/♭5. However, the blues scale can also be considered as a “dynamic” pentatonic, having two forms: one including a perfect fourth and a diminished fifth (R ♭3 4 ♭5 ♭7); and the other including an augmented fourth and a perfect fifth (R ♭3 ♯4 5 ♭7). So, perhaps the blues scale is just a dynamic attempt to implement the primitive or ideal pentatonic scale, adapting it differently in ascending and descending phrases (♯4 leading up to the perfect 5th or ♭5 leading down to the perfect 4th).
I hope you have enjoyed this presentation, and that the relations I have tried to outline will help you drawing nice pentatonic soundscapes!