The Barcelona Pavilion is a low platform, a flat roof, and a set of free-standing stone walls with nothing behind them. Slabs of golden onyx, green marble and travertine stand at angles to each other, with glass between them and shallow pools of water outside, and there is no door you can point at as the entrance. You walk in around a wall rather than through an opening. Eight cross-shaped chrome columns carry the roof, so the walls can stand wherever they like.
Germany needed a room at the international exhibition of 1929, and Mies van der Rohe and Lilly Reich gave it this: a building with nothing to exhibit but two chairs and a view framed in chrome. It stood for less than a year. In 1930 it was taken apart and the stone sold off, and for fifty years it existed only as photographs, which is how it became one of the most argued-over buildings of the century. Barcelona put it back between 1983 and 1986, on the same ground, with stone cut from the same quarries. Walk in now and there is still nothing to see but the building itself: stone, glass, water and light.