Modelling languages are intensively used in paradigms like model-driven engineering to automate all tasks of the development process. These languages may have variants, in which case, there is the need to deal with language families rather than with individual languages. However, specifying the syntax and semantics of each language variant separately is costly, hinders reuse across variants, and may yield inconsistent semantics between variants.

To attack this problem, we propose a novel, modular way to describe product lines of modelling languages. Our approach is compositional, enabling the incremental definition of language families by means of modules comprising meta-model fragments, graph transformation rules, and rule extensions. Language variants are configured by selecting the desired modules, which entails the composition of a language meta-model and a set of rules defining its semantics. This paper describes a theory that guarantees consistent semantics among all languages within the family, an implementation as an Eclipse plugin, and an evaluation reporting drastic effort reduction with respect to an enumerative approach.