Peter Quantrill

Denis Stevens was a British musicologist who, in the early 1960s, began persuading people to listen to Gesualdo's music rather than marvel at the composer with horrified fascination. One night, after rehearsing the Sixth Book of Madrigals, he was so stunned that on his way home he caught the right train going in the wrong direction.

In the summer of 1944, the head of music at French radio asked the 35-year-old Olivier Messiaen, and the Catholic writer Maurice Toesca, for a reflection on the Nativity in words and music, to be broadcast over the Christmas season. Beyond its title, there is nothing very Christmassy about the piano cycle that became Vingt Regards, which may be why Messiaen's contribution was eventually shelved.

A quick reminder: tonal music – indeed almost all western music for that matter – is built from 12 equally separated notes within the compass of an octave. Each tonality (C major, D minor, etc) uses eight of those notes in a scale: hence octave, 1-8. The root chord of each tonality (also called key signature) contains the first, third, fifth and eighth notes of the scale, 1-3-5-8.