The Keystone World is the world that we inhabit. The Keystone World is home to one of the two nexuses of reality, the Rose (the other being the Dark Tower itself). Time only flows in one direction in this world. There can be no travelling backward in time to correct errors. The number 19 is closely connected to the Keystone World. The number crops up many times when Stephen King writes a work that is set in the Keystone World.