Music Theory Tip: Tetrachords

by guitar instructor: Todd Lefever

There are a number of different ways in which to learn the notes that make up a scale or mode:
Interval Construction

Each type of scale or mode has a different intervallic pattern. An interval is the amount of distance between two notes. The most commonly used scales and modes are made up of whole step and half step intervals (a half step is equal to the distance of one fret on the guitar). For example, a major scale is a series of 8 consecutive notes (7 notes plus an octave) that uses the following intervallic pattern: whole-whole-half-whole-whole-whole-half
Scale Formula

Another way to learn a scale or mode is by remembering the scale formula that relates the intervals contained within the scale to the intervals from the major scale. For example, the natural minor scale has the formula: 1 2 b3 4 5 b6 b7.
Tetrachords

Yet another way to look at the construction of a scale or mode is by dividing it into two equal pieces called tetrachords. A tetrachord is a collection of four scalar-adjacent tones. (The term tetra is Greek for four).

Types of tetrachords:

A major tetrachord has a pattern of whole-whole-half.

A minor tetrachord has a pattern of whole-half-whole.

A phrygian tetrachord has a pattern of half-whole-whole.

A lydian tetrachord has a pattern of whole-whole-whole.

A harmonic tetrachord has a pattern of half-1½ steps-half.

A diminished tetrachord has a pattern of half-whole-half.

Each of the seven major scale modes is made up of two tetrachords:

Ionian: Major Tetrachord –whole– Major Tetrachord

Dorian: Minor Tetrachord –whole– Minor Tetrachord

Phrygian: Phrygian Tetrachord –whole– Phrygian Tetrachord

Lydian: Lydian Tetrachord –half– Major Tetrachord

Mixolydian: Major Tetrachord –whole– Minor Tetrachord

Aeolian: Minor Tetrachord –whole– Phrygian Tetrachord

Locrian: Phrygian Tetrachord –half– Lydian Tetrachord

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