Greater Perfect System

The basic musical scale unit of ancient Greece was the tetra chord meaning literally four strings.  The first and fourth notes of the tetra chord were always tuned to the interval of a perfect fourth (diatessaron) but the tuning of the other strings depended on the genus and mode of the music.  In the ancient Greek system notes of a scale were arranged in descending order.

In the diatonic genus the tuning of the other intervals comprised two tones and a semitone.  The chromatic genus comprised a minor third (three semitones) and two semitones.  The enharmonic mode comprised a major third (two tones) and two quarter tones. 

Pythagoras created the diatonic scale; dia tonic, meaning having a whole tone between the two tetra chords, which constitutes an octave also called a diatonic tetra chords.
This was further extended above and below by the addition of two conjunct tetra chords which each shared one note with the existing tetra chords.  Theorists added a further tone to the bottom of the series to complete a two octave span.  This two octaves system was called the Greater Perfect System. 

The above scale has semi-tone intervals between C and B and F and E. The others intervals are whole tones. A different scale pattern, modes, in terms of the position of tone and semitone intervals may thus be produced by traversing the scale from different starting points.

The seven possible modes are given in the following table.  Unspaced letters show semitone intervals.

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