ORGAN²/ASLSP is an organ work by John Cage, adapted in 1987 from a piano piece of 1985 whose title abbreviates the instruction to play as slowly as possible. Because Cage left the actual tempo open, the John Cage Organ Project in Halberstadt, Germany, decided to realise it over 639 years in the St Burchardi church, a span matching the time between the building of a historic Halberstadt organ in 1361 and the year 2000, when the project began.
The performance started on 5 September 2001, Cage’s 89th birthday, with a silent opening; the first audible chord sounded on 5 February 2003. A purpose-built organ, fed with air by an electric blower, sustains each chord for months or years, and pipes are added or removed only on scheduled dates. A change in the summer of 2026 brought the sounding notes from seven to eight; the next is set for 5 October 2027, and the piece is due to end on 4 September 2640.