Common Carriage is a term often used interchangeably with Open Access and Third Party Access but which is in fact more specific. In a common carriage system all applicants for capacity (for instance in a pipeline or store) are given access on equal terms. If the total volume requested exceeds available capacity, the usage of all parties is reduced pro rata: capacity is rationed among users. Under Open Access, if applicants seek more capacity than is available, capacity is apportioned on a first come, first served basis or a pay-to-book system which allows capacity to be contracted for and traded just like renting and subletting space in a building. A pipeline or store owner who wishes also to use some capacity for itself must, under Open Access, do so through an arm’s-length affiliate whose commercial relations with the capacity provider are transparently identical to those of other users. Open Access is required on US interstate pipelines, where it is known as Contract Carriage. In Europe the term Open Access is sometimes used loosely as synonymous with Third Party Access, to define the right of “third parties” to use the pipeline or equipment of another company. In its general form it encompasses Open Access, Common Carriage, Negotiated Access and Regulated Access. One aspect of “Third Party Access” is that it does not in itself imply any solution to the issue of discrimination between the parties using the facilities. See also Limited Access.