The law of rent was formulated by David Ricardo around 1809, and presented in its most developed form in his magnum opus, On the Principles of Political Economy and Taxation. This is the origin of the term Ricardian rent. Ricardo's formulation of the law was the first clear exposition of the source and magnitude of rent, and is among the most important and firmly established principles of economics.
John Stuart Mill called it the "pons asinorum" of economics.
The law of rent states that the rent of a land site is equal to the economic advantage obtained by using the site in its most productive use, relative to the advantage obtained by using marginal (i.e., the best rent-free) land for the same purpose, given the same inputs of labor and capital.
Ricardo formulated this law based on the principles put forth by Adam Smith in Wealth of Nations.
"The rent of land, therefore, considered as the price paid for the use of the land, is naturally a monopoly price. It is not at all proportioned to what the landlord may have laid out upon the improvement of the land, or to what he can afford to take; but to what the farmer can afford to give." — Adam Smith, An Inquiry into the Nature and Causes of the Wealth of Nations, Book I, Chapter XI "Of the Rent of Land"
Ricardian rent should not be confused with contract rent, which is the "actual payments tenants make for use of the properties of others." (Barlow 1986). Rather, the law of rent refers to the economic return that land should accrue for its use in production.
Being a political economist, Ricardo was not simply referring to land in terms of soil. He was primarily interested in the economic rent and locational value associated with private appropriation of any natural factor of production. The law of rent applies equally well to urban land and rural land, as it is a fundamental principle of economics.
Ricardo noticed that the bargaining power of laborers can never dip below the produce obtainable on the best available rent-free land, because whenever rent leaves them with less than they could get on that free land, they can simply move to the new location. The produce obtainable on the best available rent-free land is known as the margin of production. Since landlords have a monopoly over a given location, the only limiting factor for rent is the margin of production. Thus, rent is a differential between the productive capacity of the land and the margin of production.
Note that Ricardo's original formulation assumes that the best quality land would be the first to be used in production, and that goods are sold in a competitive, single price market.