Music theory / pentatonic scales
Pentatonic scales: the five-note scale that runs half of Western music
A pentatonic scale has five notes per octave. Two main flavors: major pentatonic (like the major scale with the 4th and 7th removed) and minor pentatonic (like the natural minor with the 2nd and 6th removed). Simpler than the major scale, more forgiving to improvise over, and older than tonal harmony itself.
Five-note scales appear in folk music from Scotland to China to sub-Saharan Africa, long before Western tonal harmony formalized. The reason: with only five notes, most combinations sound consonant. Improvise over a I-IV-V progression using the major pentatonic and you cannot pick a wrong note.
Major pentatonic in C
Take C major: C D E F G A B. Remove F and B (the two notes that create half-step tension with the tonic and dominant). You are left with C D E G A. That is C major pentatonic. On piano, the five black keys form F# major pentatonic.
Minor pentatonic in A
Take A natural minor: A B C D E F G. Remove B and F. You are left with A C D E G. That is A minor pentatonic, the blues rock scale. Add the flatted 5th (E♭) and you have the blues scale.
Guitar box shape (minor pentatonic, position 1)
A minor pentatonic in position 5 (starting at fret 5 on the low E) is the most drilled scale in rock. Box shape: E string 5-8, A 5-7, D 5-7, G 5-7, B 5-8, high E 5-8.
Piston notes that the pentatonic scale predates the seven-note diatonic system historically and remains the harmonic basis of most folk traditions. Its avoidance of half-steps is not accidental; the scale was selected across cultures for its consonance with drone accompaniment, which itself predates functional harmony by centuries.
Piston, Harmony, 5th ed, appendix on scales.
Why pentatonic works over I-IV-V
The five notes of the major pentatonic (C D E G A in C) are all consonant with the I chord (C E G), the IV chord (F A C, three shared tones), and the V chord (G B D, two shared tones with the 4th note pending). No half-steps means no clashes with any of those chords. This is why the major pentatonic is the first improvisation scale most teachers introduce.
Practice with the circle
Use the circle of fifths to find any major pentatonic (start from the major scale, remove the 4th and 7th) or minor pentatonic (relative minor of the major pentatonic).
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