Lets explore the interval structure of the modes.
Tones and Semitones
Scales are sets of notes arranged into tones and semitones, also known as whole-steps and half-steps. Simply put, a tone (whole-step) is a distance of two frets, while a semitone (half-step) is a distance of only one fret.
For those that know a little about piano, a tone is one key apart, while semitones are adjacent keys.
The Interval Structure of The Major Scale
The notes of the Major scale (Ionian mode) are spelled out as Tone, Tone, Semitone, Tone, Tone, Tone, Semitone. Since a tone is two frets, and a semitone is one fret, a major scale could look like this, along a single string. (Of course, we would rarely use a scale laid out on a single string – but mapping it to the fretboard like this does make it easier to see the intervals (fret-distance) between each note.)
Realise that I haven’t just pulled this fretboard map out of thin air. It’s been taken from the regular C Ionian/Major scale (covered in Modes Part 2). In case you can’t see the connection, here is the full fretboard map with the other notes ‘greyed-out’.
The Interval Structure of The Modes
As we learned in Modes Part 2, a mode is little more than a major scale starting in a different place. This means that finding the interval ‘formulas’ of the other modes is as simple as starting the Tone, Tone, Semitone, Tone, Tone, Tone, Semitone formula at a different place.
For instance to get the Dorian mode formula, we will start at the second interval of the formula, since Dorian is the second mode of major. This would give us the Dorian interval structure as being Tone, Semitone, Tone, Tone, Tone, Semitone, Tone. Similarly, the formula for Phrygian, the third mode, would start at the third interval of the major scale formula, giving us Semitone, Tone, Tone, Tone, Semitone, Tone, Tone.
I’ve provided a table below for easy reference, but you should make sure that you can work out the formulas yourself.
And in case you’re a more visual person, here are those same modes mapped out along single strings.