What is "mi-parti"

Mi-parti is an orchestral work by the Polish composer Witold Lutosławski, composed from 1975 to 1976 on a commission from the City of Amsterdam for the Concertgebouw Orchestra. The name broadly means in two equal but different parts, referring to the treatment of the material rather than the large-scale structure of the piece. First performance: October 22 X 1976, Rotterdam, Concertgebouw Orkest, the composer conducting.