## Friday, June 19, 2009

### Tetrachords

The tetrachord is a group of four tones spanning a perfect 4th. It is the basis of the Diatonic system, and it is the basis of the Fretography method.

A perfect 4th is an interval of two whole-steps and one half-step. For instance; C-D-E-F, which is based on the intervals; W W H (W=Whole-step, H=Half-step). So the first four notes of a major scale (in this case; C major) form a tetrachord. The next four notes in the scale; G-A-B-C are based on the same interval pattern; W W H, so it is a tetrachord identical in structure to the first. There is a whole-step between the two tetrachords, resulting in the pattern below;

tetrachord I - tetrachord II
C (W) D (W) E (H) F (W) G (W) A (W) B (H) C

This interval pattern is the same for every key. It can be expressed in terms of scale degree where a Roman numeral is used for each tone instead of an alphabet letter;

tetrachord I - tetrachord II
I (W) II (W) III (H) IV (W) V (W) VI (W) VII (H) I

One way to apply this principle to the fretboard is to locate the positions of the half-steps within a key. By memorizing the pattern of half-step positions on the fret board, all the note positions can be learned more easily since the remaining notes will all be based on whole-steps.

In other words, simply by knowing that half-steps are III-IV (E-F) and VII-I (B-C) you have learned the precise positions of four of the seven tones of the key, and the other three tones pretty much fall into place.

In the next post we'll look at the position of the half-steps on the fretboard.

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