Domain-specific languages are constructed to provide modelling capabilities tailored to a specific domain. Sometimes, languages are developed many times, typically to support application in a new context. In doing so, recurring patterns and commonalities as well as variations across the evolving set of languages can be identified. This paper introduces the concept of a domain-specific metamodelling language, which codifies such commonalities and provides concepts and logic for expressing the variations. The challenges and difficulties of using domain-specific metamodelling languages are identified. We illustrate the concept with examples from different domains.