ral systems theory". Languages for hard- ware and software systems modelling should support the specific characteristics for those very general system classes. However, since there exists neither agreed classification of system domains nor science for "general systems", and since soft- ware systems are dominant components in systems, commercial modelling meth- ods are practically developed only for modelling the software implementation of systems. 1.1 Why Another Modelling Language? General systems Domain and technology independent systems Processing systems Domain specific system (e.g. a telecom system) Domain specific and technology independent systems Information systems Technology specific systems Software Hardware isApartOf isA or inherits implements systems systems Fig. 1.1 A taxonomy for systems domains At best, efforts are sometimes made to extrapolate languages for modelling soft- ware into the area of domain-specific and general systems. This is a bottom-up approach that is far from optimal, since there are numerous domain specific sys- items. Within each domain there are domain-specific concepts, constructs and pat- terns. These must be supported by domain-specific notations, in order for the modelling technique to be really useful. The properties one wants to describe for domain-specific systems also include physical and spatial properties, and types of interfaces other than just interfaces between software entities. Domain-specific modelling is therefore an issue that must be dealt with separately from software system modelling. To support the development of domain-specific modelling tools, a meta-language that can be used for specifying the semantics and syntax of domain specific modelling languages is required. Having access to a formal speci- fication of a domain-specific language also provides a good possibility to map the constructs of the language on the constructs of a software modelling languages, i.e. to apply a top-down approach on modelling languages. The state-of-the-art of domain-specifi