Watch the A Dorian Mode online guitar lesson by Brad Carlton from Guitar Lab: Dorian Dominance
A heptatonic arpeggio is a very practical way to understand the harmonic implications of a seven note scale. When you view a scale, you are playing the notes in seconds which means each adjacent note in the scale. A heptatonic arpeggio simply takes the same seven notes in the scale but aligns them in thirds which means every other note of the scale. This reveals tertiary harmony, meaning chords build in thirds, and thus shows the largest chord that could be built from any scale.
The formula for the Dorian mode is: 1, 2, b3, 4, 5, 6, b7. If we align these notes in thirds, we will get the resulting formula: 1, b3, 5, b7, 9(2), 11(4), 13(6). This would be the formula for a minor 13 chord.
I have shown various layouts on the fingerboard for playing this minor 13 arpeggio. Diagrams 60 through 65 display these layouts.