Certain limitations will be caused by the deliberate action of some group of persons to render some thing or some service scarce, as when competitors restrict output in agreement, or a trade union restricts entry to a trade; and this may be distinguished as deliberate or intentional monopoly. The theory of monopoly is, as a rule, confined in economic writings to this species. It is to be noticed, however, that the term monopoly describes a relation between things or services and persons; and hence natural monopoly applies as a rule only to personal qualities restricted by nature. A natural scarcity of objects cannot be converted into a monopoly attaching to persons unless an institutional element is added as well. Besides advantage due to monopoly, however, it is important to notice certain other forms of advantage, which are usually described as advantages in bargaining power.