Organ improvisation is a remarkable art, whether or not accompanying a silent film. In any of France's great cathedrals, the foremost musical experience is to hear the "titulaire" (essentially the headline organist) improvising on whatever theme they've selected for the day - sometimes chant, sometimes a hymn tune, sometimes something entirely frivolous and inappropriate that takes on a life of its own.

Even here in the UK, where it's not such a big thing, there's often an enjoyable few minutes at evensong where the organist improvises in the gap between finishing the prelude and the choir processing in. But France does it like nowhere else. One of the finest musical experiences I've ever known was Olivier Latry, titulaire at Notre Dame, giving a recital at Worcester Cathedral. After the appointed recital, he performed a 20-minute improvisation on the hymn chosen for evensong earlier that day (Herbert Howells' "Michael"), which he had never heard before. Superb yet entirely ephemeral - like most improvisations, it was never recorded.