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82 Hertz: Relating pentatonic and diatonic scales

Relating pentatonic and diatonic scales

Many popular songs are based on the "Major" scale, which is a type of diatonic scale. Many of them use the just the three major chords associated with that scale, others will add one or more of the relative minor chords.

Knowing how this all relates provides a good basic understanding of how to quickly select which notes to use in an improvisation that could closely match the song's melody.

Diatonic scales

A diatonic scale is the kind of scale you're most familiar with. The C major scale, which can be made from the white keys on the piano, is one of them. More precisely:

  • it has eight notes, including a repeated octave note
  • the note intervals includes five whole steps and two half steps
  • the two half steps are separated from each other by either two or three whole steps

The Major scale, the one made from the white keys when the root note is C, is also called the Ionian scale. If the root starts on a note other than C it is no longer a "major scale", and is considered a different "mode" than Ionian. For this discussion, we don't need to know any more than that about modes.

When the major scale is played starting on the note C, it is a C Major scale. Songs based on this scale are often considered to be in the key of C.

Pentatonic scales

For the rest of this page I will only be talking about the C Major scale. The concepts apply to any major scale.

There are three major chords in a diatonic scale. For the key of C they are C, F and G. Each of these chords can have a major pentatonic scale associated with it. For the C chord, the notes of the C major pentatonic scale are made from notes 1, 2, 3, 5 and 6 or the C major scale.

Those same intervals, the number of "half-steps" between the notes, is the same for every Major Pentatonic scale. Wikipedia has a section that describes various ways of obtaining the notes in the major pentatonic scale.

That description points out that there are three different major pentatonic scales can be constructed by removing notes from the diatonic major scale. Remove the:

  • forth and seventh degree to get a C major pentatonic
  • third and seventh degree to get an F major pentatonic
  • first and forth degree to get a G major pentatonic

Let's look at it

We can take this knowledge of the diatonic and pentatonic scales and look at how they relate on a fretboard. At the same time, we'll look at how the major chords and the major pentatonic scales work together. To keep things simple, we'll continue to only talk about the key of C.

The C major diatonic scale has three major pentatonic scales within it: C, F and G.

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Let's see what they the chords and scales look like next to each other.

A few reminders about the fretboard images…

The larger circles are part of both the chord and the pentatonic. The smaller notes are only in the pentatonic pattern; they may still be notes that are common with the chord but they aren't used in that particular form. The large circled notes are the roots.

The same patterns are shown in both a vertical and a horizontal display because some people have trouble with one form or the other.

Because this can be modified to be used any key, the chords and patterns are shown as barred chords. In the key of C, pretend that the lowest notes (on the left or top of the images) are the guitar's nut.

C major pentatonic

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F major pentatonic

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G major pentatonic

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C major diatonic

All of the notes in the above pentatonics are in the C major scale. There are no notes outside of the major scale and there are no major scale notes that are not included in at least one of the pentatonics. This fretboard image is the Phygian position.

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Using the patterns

As stated at the start of this page, most popular music is based on a diatonic scale and probably also uses the three major chords (I, IV and V) that are in that scale.

Each major chord also has a relative minor (vii, ii and iii). The note pattern for a major pentatonic and its relative minor are identical, so the pentatonic patterns above cover 6 of the seven chords in the scale.

The specific pentatonic patterns used in this example are nice for another reason: you can play all 3 (6 if you count the relative minor) patterns in just four frets. There's no stretching involved, just one fret per finger.

There's another place where that can be done; as a hint, the root (I) chord uses a barred E pattern.

Rules are made to be broken, but in general the notes most frequently used in a melody in popular music are: the chord tones, followed by the pentatonic, followed by the other notes in the diatonic scale. Again, there are always exceptions. But if you're trying to pick out a melody or learn a lead pattern, this is a good spot to start.

The most common exception to the above rule is when a seventh chord is used that is not in the scale. In the key we're been talking about, a C7 would be one of those chords.

But that's a discussion for a different article.

There is an enormous amount of information on the web about diatonic scales and modes, so there is no point in repeating it here.