Complex sentence: In 1961, Ligeti wrote perhaps the largest cluster chord ever--in the orchestral "Atmosphères", every note in the chromatic scale over a range of five octaves is played at once (quietly).
Split into simpler sentences: In 1961, Ligeti wrote perhaps the largest cluster chord ever. In the orchestral "Atmosphères", every note in the chromatic scale over a range of five octaves is played at once (quietly).