Grist: Architecturally, a keystone is the wedge-shaped piece at the crown of an arch that locks the other pieces in place. Without the keystone, the building blocks of an archway will tumble and fall, with no support system for the weight of the arch. Much of the United States climate movement right now is structured like an archway, with all of its blocks resting on a keystone - President Obamas decision on the Keystone XL pipeline.
This is a dangerous place to be. Once Barack Obama makes his decision...