Cities have to be studied at different scales. We can look from a wider perspective (upscale) or with greater detail (downscale). For example, from an upscale point of view we find that the majority of cities form part of a region with many other cities and create a system of centralities. However, from a downscale perspective, at city scale, we find other centralities, such as the airport, business district, mediaeval city, university quarter, etc.

If we centre on Barcelona, this theory is not particularly reflected as it is a very compact city, continuous and efficient. This is because it is a city with one of the highest population densities in the world.