A primate city is by far the largest and most influential city in a country. It serves as the center of political power and culture, influencing trends in art and style across the entire country. In addition, a primate city is a hub for national economics and development.

The primate city concept was first developed by Jefferson, who theorized that a country should have one city that dominates the nation. While his theory has been largely debunked, the idea of a central urban area that dominates the country's growth and development is still prevalent in many countries.

In order for a city to be considered a primate city, it must have at least double the population of the second-largest city in the country. This rule was designed to provide an even distribution of the population among the various cities in a country.

While the rank size rule has not been strictly followed in every country, it provides a good starting point for understanding how cities grow and develop around the world. There are several factors that influence whether a city is considered a primate city, including the country's history, its development, and how it grows and expands over time.

For example, while the UK is a densely populated country, London has a population of over 9.5 million, while Birmingham's population hovers at less than a million, which would disqualify it from following the rank size rule. In contrast, the US's biggest city, New York, has a population of over 21 million, which would qualify it as a primate city.