Pli selon pli (Fold by fold) is a piece of classical music by the French composer Pierre Boulez. It carries the subtitle Portrait de Mallarmé (Portrait of Mallarmé). It is scored for a solo soprano and orchestra and uses the texts of three sonnets of French symbolist poet Stéphane Mallarmé and single lines from two of his other poems. At over an hour, it is Boulez's longest work.