The orchestral work Mi-Parti was commissioned by the city of Amsterdam for its Concertgebouw Orchestra, which gave the work its premiere under Lutoslawski's direction on October 22, 1976. Lutoslawski found the piece's unusual title in a dictionary; its definition is "composed of two equal but unlike parts." The title doesn't refer to the work as a whole, however, but only to the melodic phrases played in the work's slow opening.

In this 15-minute composition Lutoslawski contrasts what he calls "warm" colors, using intervals of thirds and sixths, with the "cold" colors of major seconds, tritones, fourths, and fifths. The warm colors are employed at first, in a spacious and sensuous opening combining string glissandi (slides) and sustained tones. Clarinet, French horn, and other winds join in with short melodic motifs. The orchestration becomes denser as Lutoslawski starts to introduce the "cold" colors. The music becomes louder and more chaotic as pizzicato (plucked) strings, scurrying figures in the piano, and jagged brass fanfares appear and build to a wild and noisy climax. The colors then start to change again; dense string sonorities gradually ascend to the stratosphere, and sustained tones fade to a quiet and shimmering ending.

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