The spectralist Grisey wishes to use both harmonic and inharmonic spectra, and the inharmonic are either compressed (4 overtone series where the intervals are slightly diminished) or they are extended (4 overtone series where the intervals are slightly augmented).
The 4 harmonic series (with the fundamentals on F#, Eb, C and A – making a diminished 7th-chord) have the 4 quartertones as coinciding pitches (as their 11th and 13th partials), and these same pitches are also found in the inharmonic series. In this way, the quarter-low C, the quarter-high D, the quarter-high F and the quarter-low A are central pitches through the whole work.
Grisey chose to octavate some of the pitches, because he wished to have the possibility of microtonal sounds in different ranges of the piano.
