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III.3Part III, Tonality
The Tetrachord
You'll see how a major scale splits into two identical tetrachords, and why this lets one scale lead to the next.
A tetrachord, from the Greek “tetra”, four, and “chordē”, string, is a series of four stepwise notes. The scale, with its eight notes, contains two of them: the lower tetrachord, made of the four lowest notes, and the upper tetrachord, made of the four highest.
Four things to notice
- The two tetrachords are exactly alike: each one is made of two consecutive whole steps followed by a diatonic half step.
- The first note of the lower tetrachord is the tonic, and the first note of the upper tetrachord is the dominant.
- The two tetrachords are separated by a major second, here F-G.
- The two outer notes of each tetrachord, C-F and G-C, form a perfect fourth.
The consequence
Because the two tetrachords are identical, two things become possible:
- The lower tetrachord can become the upper tetrachord of a new scale, which you then complete with a new tetrachord below.
- The upper tetrachord can become the lower tetrachord of a new scale, which you then complete with a new tetrachord above.
This is the mechanism that will generate all the scales with sharps, as you'll see in the next lesson, and all the scales with flats.
WorkshopScale BuilderFull screen
Look at the two tetrachords of other major scales: the structure never changes.
Steps: 1 1 ½ 1 1 1 ½ (1 = whole step, ½ = half step, 1½ = augmented second)
- Scale
- D major
- Key signature
- 2 sharps: F♯, C♯
- Degrees
- 1. tonic · 2. supertonic · 3. mediant · 4. subdominant · 5. dominant · 6. submediant · 7. leading tone
- Tetrachords
- D E F♯ G | A B C♯ D: two identical tetrachords (whole, whole, half) a whole step apart
Exercisein the method's spirit
In the D major scale, identify the lower and upper tetrachords, the interval between them, and the interval formed by the outer notes of each.
Check your understanding
7 questions1.
A tetrachord is:
2.
In the major scale, each tetrachord is made of:
3.
What interval separates the two tetrachords of a major scale?
4.
The outer notes of a tetrachord form:
5.
On which degree does the upper tetrachord begin?
6.
What is the upper tetrachord of D major?
7.
What is the lower tetrachord of F major?