'Well-tempered'?

And, yes, first of all, why 'well-tempered'? What then does this familiar, yet somewhat mysterious, expression cover? The explanation calls for a short historical reminder. We must begin by taking into account an ineluctable physical fact: it is impossible to divide the scale into twelve perfectly true semitones and therefore to tune an instrument of fixed tones, such as the harpsichord or organ, in as many physically pure intervals. For quite some time, musicians, physicists and mathematicians, instrument makers and even philosophers wondered about the various questions raised by the tuning of fixed-tone instruments. If we begin by tuning the first intervals without oscillations – i.e., physically true –, this will not be the case for the following ones. A keyboard thus tuned only permits the execution of music in a few simple keys, with few possibilities of modulations. Owing to this, the composers' imagination found itself curbed, and music's expressive resources considerably limited. As for other keys, they become impracticable, certain intervals sounding 'false'. For a long time, one spoke of the 'goat key' or the 'wolf fifth', to say nothing of a diabolus in musica that could really scream to the point of frightening.

It was therefore necessary to find a stratagem, consisting of cheating very slightly on certain intervals that were too large and therefore too harsh to the ear, softening them, i.e., by 'tempering' them.

Every prelude and fugue thus illustrates a different effect of the human being – in this case, of Johann Sebastian Bach in person who, in the course of these pages, creates a musical self-portrait in small strokes and a veritable treatise of the soul's passions in music.