"[T]his is one of my favorite pieces of Bach... the Prelude in G minor from Book 2 of the preludes and fugues The Well-Tempered Clavier... [T]here is a huge quantity of maths in Bach. Because... the tuning system enabled him to write in every key, for the first time ever... he wrote a prelude and fugue in every key. There are 12 keys on the piano. He wrote in major and minor, so that made 24 preludes and fugues. Then he got really excited, and did it again, so that made 48..."