What is an American city?

For many years I have argued that in the decades after the Second World War, economic, demographic, and spatial transformations in the United States resulted in an urban form unlike any other in history. Recently, I realized that in one important way this formulation of recent urban history misleads, for it reports the outcome of history as singular when it should be plural. "Form" should be "forms"—what we have today is an unprecedented configuration of urban places that calls into question the definition of city itself.