A small organ in eastern Germany is the headliner of the world's slowest concert. It has been playing the same piece of music by composer John Cage at a glacial pace for nearly 25 years. A rare chord change on Wednesday - from seven notes to eight - represented the completion of about four per cent of the 639-year endeavour of the John Cage Organ Project Halberstadt. A chord change, done manually, means the sound of the organ pipes changes either because new sounds are added or existing sounds end. Then, an eighth pipe was slotted into the small organ, bringing with it a new sound that reverberated throughout the church.