Some local products are available only in large cities. If the per-capita demand for a product is low relative to the scale economies in production, it will take a large city to generate enough demand to support a firm. For example, the per-capita demand for opera is relatively small, so it may take a million people to support an opera company. As a result, we will find opera companies in large cities but not small ones. Similarly, the per-capita demand for brain surgery is low relative to the scale economies in production, so brain surgeons operate in large cities.