Membrane structures are spatial structures made out of tensioned membranes. The structural use of membranes can be divided into pneumatic structures, tensile membrane structures, and cable domes. In these three kinds of structure, membranes work together with cables, columns and other construction members to find a form. Membranes are also used as non-structural cladding, as at the Beijing National Stadium where the spaces between the massive steel structural members are infilled with PTFE coated glass fiber fabric and ETFE foil. The other major building on the site, built for the 2008 Summer Olympics, is the Beijing National Aquatics Center, also known as the Water Cube. It is entirely clad in 100,000 square metres of inflated ETFE foil cushions arranged as an apparently random cellular structure.