Feature modelling has become perhaps the most popular method for representing variabilities and commonalities in software product families. A large number of feature modelling methods and supporting tools have been reported. The conceptual foundation of feature models remains vague, a fact that severely undermines the usability of feature models. Therefore, we introduce Forfamel, a rigorous conceptual foundation for feature modelling. Forfamel synthesises existing feature modelling methods in the sense that it covers most concepts and constructs found in existing feature modelling methods. In addition, Forfamel includes a few additional constructs that may prove to be useful. We show the novel contribution of Forfamel by reflecting it against previous feature modelling methods and arguing for its underlying design decisions