Technical writing explains and, in contrast to other kinds of writing, insists on the visual. Explanation and visualization are mutually dependent, because explanation combines “description,” the observable facts, and “diagram,” the graphic paradigm of the relations that obtain among these facts. The technical writer's principal task, then, is to make explicit, by using appropriate spatial, temporal, and logical signals, one of three diagrams—schematic, flowchart, tree—that define the basic modes of explanation—object, process, and logical hierarchy. Where the novelist submerges the diagram in metaphoric layers, the technical writer strips them away and surfaces the diagram.