The first notes in the longest and slowest piece of music in history, designed to go on for 639 years, are being played on a German church organ on Wednesday.

The three notes, which will last for a year-and-a-half, are just the start of the piece, called As Slow As Possible.

Composed by late avant-garde composer John Cage, the performance has already been going for 17 months - although all that has been heard so far is the sound of the organ's bellows being inflated.