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Polyurethane (PUR) foam of low spatial filling (P1 < 10%) is a polymer-gas composite. Its structure is a spatial network of polymeric struts, nodes, and films that are organized into cells. The cellular structure is well-suited for mathematical modeling to determine effective physical and mechanical properties in connection with its symmetry of structure. The PUR foam capacity of free rise in an open mold of comparable planar dimensions leads to the symmetry mode of transtropy. The cover by Ilze Beverte shows light microscopy images of transtropic, free rise PUR foam with a density of 35 kgm−3, P1 < 3%. The image plane is perpendicular to the foam symmetry axis, collinear to the rise direction. DOI: 10.1002/app.39477