Club goods (also artificially scarce goods or toll goods or collective goods) are a type of good in economics, sometimes classified as a subtype of public goods that are excludable but non-rivalrous, at least until reaching a point where congestion occurs. Often these goods exhibit high excludability, but at the same time low rivalry in consumption. Thus, club goods have essentially zero marginal costs and are generally provided by what is commonly known as natural monopolies. Furthermore, club goods have artificial scarcity. Club theory is the area of economics that studies these goods. One of the most famous provisions was published by Buchanan in 1965 "An Economic Theory of Clubs," in which he addresses the question of how the size of the group influences the voluntary provision of a public good and more fundamentally provides a theoretical structure of communal or collective ownership-consumption arrangements.