This Recommendation provides information on the functions of a terminal adaptor from both the conceptual and practical points of view. The expression “Terminal Adaptor” is commonly used to refer to a stand-alone item of equipment (or alternatively a printed circuit card plugged into a terminal) that allows a non-ISDN terminal to be connected to an ISDN user-network interface. Formally though, Terminal Adaptor (TA) is the name of one of the (conceptual) Functional Groups defined in Recommendation I.411. The TA Functional Group contains functions that allow a TE2 (non-ISDN) terminal to use the services provided by an ISDN. The popular understanding of the term TA is thus well aligned with the theoretical definition. This Recommendation coordinates information from other Recommendations, expands on topics only briefly discussed elsewhere, describes the relationship between Terminal Adaptors and Modems, and provides tutorial material, and guidance on future enhancements and applications.