Scope/Objective Recommendation ITU-T Z.101 defines the basic features of the Specification and Description Language. Together with Recommendations ITU-T Z.100, ITU-T Z.102, ITU-T Z.103, ITU T Z.104, ITU-T Z.105, and ITU-T Z.106 and ITU-T Z.107, this Recommendation is part of a reference manual for the language. The language defined in this document covers the essential features of the language, which is further defined in other Recommendations in the ITU-T Z.100 series. Coverage The Specification and Description Language has concepts for behaviour, data description and (particularly for larger systems) structuring. The basis of behaviour description is extended finite state machines communicating by messages. Data description is based on data types for values and objects. The basis for structuring is hierarchical decomposition and type hierarchies. A distinctive feature of the Specification and Description Language is the graphical representation. This Recommendation covers the main features of the language such as agent (block, process) type diagrams, agent diagrams for structures with channels, diagrams for extended finite state machines and the associated semantics for these basic features. The concrete grammar given is the graphical representation. The alternative textual programming representation is given in Recommendation ITU-T Z.106. The concrete grammar in this Recommendation gives a canonical syntax, which is extended in Recommendation ITU-T Z.103 to a syntax that is easier to use. The basic part of the language given in this Recommendation does not include the details of expressions, data definitions and action language, which is defined in Recommendation ITU-T Z.104 and for object oriented data ITU T Z.107. The features of the language defined in Recommendation ITU-T Z.102 make the language more comprehensive. Applications The Specification and Description Language is applicable within standard bodies and industry. The main application areas for which The Specification and Description Language has been designed are stated in ITU-T Z.100, but the language is generally suitable for describing reactive systems. The range of application is from requirement description to implementation. The features of the language defined in ITU-T Z.101 allow basic models to be defined and provide a basis for other features defined in other Recommendations in the ITU-T Z.100 series.