The world of the novel is a mutable one, in which characters have imperfect and incomplete knowledge of the world at large. At first, many of the characters view the world in strict and simple binaries between the Downside and the Topside. As characters learn about the world, their perspectives shift dramatically. Though Downsiders present themselves as separate from Topsiders, Downsiders depend on Topsiders, with Talon snapping at Railborn, “Where do you think lightbulbs come from? And batteries, and pens, and conditioning shampoo?” (27). In the mutable world, objects flow from the Topside to the Downside, where Downsiders can use or repurpose them. For example, Downsiders turn socks into currency. Talon gives Lindsay a sock in exchange for the medicine, and Downsiders leave socks when they take a library book. Likewise, Topsiders rely on Downsiders for utilities such as gas, water, and electricity, and New York City grinds to a halt when Railborn demands that these utilities be cut off as punishment for the Topside’s “incursion.” The flow of goods and services between the Topside and the Downside highlights their interdependence.

Although Downsiders would prefer to maintain a strict demarcation between the two worlds, in reality the boundary between the two is porous.