Many industries are geographically concentrated. Many mechanisms that could account for such agglomeration have been proposed. We note that these theories make different predictions about which pairs of industries should be coagglomerated. We discuss the measurement of coagglomeration and use...

"Urban proximity can reduce the costs of shipping goods and speed the flow of ideas. Improvements in communication technology might erode these advantages and allow people and firms to decentralize. However, improvements in transportation and communication technology can also increase the...