This International Standard defines, in an abstract way, the externally visible Basic Class Virtual Terminal Service within the OSI Application Layer in terms of a) a model defining the interaction between users of the service; b) the primitive actions and events of the service; c) the parameter data associated with each primitive action and event; d) the relationship between, and the valid sequences of, these actions and events. The service defined in this International Standard is that which is provided by the OSI Basic Class Virtual Terminal Protocol (in conjunction with the Association Control Service Element and the Presentation Service) and which may be used by any user including other Application Service Elements. The relationship between the standards for Virtual Terminal Service, Virtual Terminal Protocol, ACSE, Presentation Layer Service and the user of the Virtual Terminal Service is shown in figure 1. This International Standard also defines two standard default virtual terminal environment profiles and describes the form of registered virtual terminal environment profiles and control objects. Virtual terminal environment profiles define sets of virtual terminal environment parameters for use in the establishment of virtual terminal associations and subsequent negotiation. This International Standard also defines a structure of ASN.1 Object Identifiers for the objects defined in this International Standard and for use in a register of virtual terminal objects.