This part of ISO 3386 specifies a method for the determination of the compression stress/strain characteristics of low-density flexible cellular materials up to 250 kg/m3. It also indicates a method for the calculation of the compression stress value of such materials. The compression stress/strain characteristics is a measure of the load-bearing properties of the material, though not necessarily of its capacity to sustain a long-term load. The compression stress/strain characteristics differs from the indentation hardness characteristics (as determined in accordance with ISO 2439), which are known to be influenced by the thickness and the tensile properties of the flexible cellular material under test, by the shape of the compression plate and by the shape and size of the test piece. ISO 3386-2 specifies a method for high-density materials and differs from part 1 in the following ways: It is mainly concerned with materials of density above 250 kg/m3; Compression stress values have been deleted; It does not permit the use of a cylindrical test piece.