Towards the end of the 19th Century with a second Industrial Revolution, steel became much cheaper. Architects began to use inventions like metal girders and reinforced concrete to build. An example is the Eiffel Tower in Paris. Buildings can now be built taller than ever before. We call them skyscrapers. This new technology has made us free from traditional limitations, and because of the new possibilities presented by these materials, many traditional methods of construction and ideas about style were reevaluated, replaced, or abandoned. Cheap, strong glass soon brought transparent exterior walls, especially for office buildings.