An organ in a church in Halberstadt, Germany, just changed chords for the first time in two years when a pipe was added. The organ is playing a 639-year-long song, American composer John Cage’s “Organ²/ASLSP (As Slow as Possible),” which will end in 2640, according to Smithsonian Magazine.

“In the beginning of the first part, for 17 months, you came in here and heard only the bellows,” John Cage Organ Foundation member Rainer Neugebauer told NPR. They realized later the quiet time should have lasted 28 months.

“I’m 99% sure that John Cage, if he’s sitting on a cloud somewhere, would say, ‘Oh, it’s good,’” Neugebauer recalled. “He would laugh about the mistakes we’ve made.” (AP Photo: John Cage)