Diatonic Theory - Constants of a Key

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Since this Interval sequence of the Major Scale never changes you can also look at those basic Triads/Chord we built from the C Major Scale in Part 5 as being static, constant, or consistent. Since the Interval sequence never changes this also clues us into the fact that the Triad/Chord sequence will NEVER change in a Major Key, because they are built from the consistent Interval structure of the Major Scale.

This is where the "I-ii-iii-IV-V-vi-vii-I sequence comes from, the constants of the Scale they are built 
from.

Let's look at a couple of Keys and see that this sequence of chords built from the Scale/Key is the same 
regardless of the Root of the Key.

The chords in the Key of C Major:

C - Dm - Em - F - G - Am - Bmb5 - C

The chords in the Key of G Major:

G - Am - Bm - C - D - Em - F#mb5 - G

The chords in the Key of D Major:

D - Em - F#m - G - A - Bm - C#mb5 - D

The chords in the Key of F# Major:

F# - G#m - A#m - B - C# - D#m - E#mb5 - F#

So you can see that the Name of the notes of each chord are based on the notes of the particular Major 
Scale, and that the WWHWWWH sequence is constant, and that the Triads/chords of ANY Major scale will ALWAYS be:

Major - Minor - Minor - Major - Major - Minor - Minor b5 - and Major again.

Then the sequence would start all over again since the Notes aren't changing.

Since the sequence of Intervals and Chords of a Key never change you can also see that the Mode Names and how they are placed and tied to a Key never changes either.

This is the sequence of the Mode Names within a Key:

Ionian-Dorian-Phrygian-Lydian-Mixolydian-Aeolean-Locrian-Ionian again to complete the sequence.

So, as we saw with the Major Scale Intervals and Chord sequence, the Mode Names will also be in a consistant order when dealing with a Key. Meaning Ionian will ALWAYS be the first Mode of a Major Key, Dorian will ALWAYS be the second Mode of a Major Key, Phrygian will ALWAYS be the third Mode of a Major Key, and so on.

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