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	Urban Primacy Index	Rank-Size Rule
Field	Human Geography	Human Geography
Family	Process / pipeline	Process / pipeline
Year of origin	1939	1949
Originator	Mark Jefferson	George Kingsley Zipf
Type	Index of the dominance of the largest city in an urban system	Empirical regularity and diagnostic for the size distribution of cities
Seminal source	Jefferson, M. (1939). The Law of the Primate City. Geographical Review, 29(2), 226–232.	Zipf, G. K. (1949). Human Behavior and the Principle of Least Effort. Addison-Wesley, Cambridge, MA. ISBN: 9781614273790
Aliases	Primacy Index, Primate City Index, Two-City Primacy Index, Four-City Primacy Index	Zipf's Law for Cities, Rank-Size Distribution, City-Size Rank-Size Relationship, Rank-Size Regularity
Related	4	4
Summary	The urban primacy index measures how dominant a country's largest city is relative to the cities below it in the size hierarchy. It grows out of Mark Jefferson's 1939 law of the primate city, which observed that many countries are headed by a single city far larger and more important than any other. The simplest two-city index divides the largest city's population by the second-largest's, while the four-city index compares the leading city with the combined size of the next three, giving a compact gauge of urban concentration.	The rank-size rule is an empirical regularity describing the size distribution of cities within a country or region. In its simplest form, popularized by George Kingsley Zipf in 1949, the population of a city is inversely proportional to its rank, so the second-largest city is about half the size of the largest, the third about a third, and so on. Generalized to a power law with an exponent q, it provides a compact way to summarize how evenly or unevenly population is spread across a settlement system and to diagnose urban primacy.