Building the Great Cathedrals, from PBS's Nova

  In the video that we watched in class this week (Building the Great Cathedrals, from PBS's Nova) the narrator spoke about three innovations or inventions that helped to give Gothic Architecture it's signature look. Write a 1-2 page essay on the following topic-   What are the three innovations? In your own words, how does each work to reduce pressure on the walls? And what do they allow the builders to use that gives Gothic architecture it's signature look?