Model-driven development of software-intensive systems aims at designing systems by stepwise model refinement. In order to create software product lines by model-driven development, product variability has to be represented on every modelling level and preserved under model refinement. In this paper, we propose -modelling as an generally applicable variability modelling concept that is orthogonal to model refinement. Products on each modelling level are represented by a core model and a set of -models specifying changes to the core to incorporate product features. Core and -models can be refined independently to obtain a more detailed model of the product line. Based on a formalization of -modelling, we establish conditions that model refinement and model configuration commute resulting in an incremental modeldriven development process. Keywords-Software Product Lines; Variability Modelling; Model-driven Development; Model Refinement