External representations of thought—maps, diagrams, sketches, and the like—are ancient inventions that serve thought and communication in numerous ways. A number of cognitive scientists have investigated roles these representations play in cognition (see, e.g., Donald, 1991; Larkin & Simon, 1987; Norman, 1993; Schön, 1983). They are created and used by school students, by architects and designers, by mathematicians and scientists, by musicians, dancers, and artists. People design and use diagrams to spatialize thought and make it public, to work through ideas and clarify thinking, to reduce working memory load, to communicate ideas to others, to promote collaborative work by providing an external representation that can be pointed to and animated by gestures and collectively revised. Considerable research has shown that well-designed diagrams promote thought, creativity, discovery, and communication. Diagrams can map abstract thought to space, allowing spatial reasoning to promote abstract reasoning.