The paper presents a comprehensive reference model for the digital twin in design and production engineering. The digital twin is a virtual representation of a physical product that enables the integration of design, manufacturing, and operational data throughout the product life-cycle. The authors propose a reference model based on the concept of Skin Model Shapes, which serves as a digital twin of the physical product. This model allows for the representation, implementation, and application of virtual product models in design and manufacturing. The model is designed to ensure scalability, interoperability, expansibility, and fidelity. The paper discusses the evolution of the digital twin concept, its current industrial understanding, and the challenges in its implementation. It also highlights the application of the reference model to geometrical variations management, which involves the differentiation between the real workpiece, its abstract model (Skin Model), and the virtual representation of this model. The paper concludes that a sound conceptual framework and a comprehensive reference model are essential for the successful implementation of the digital twin vision. The reference model enables the clear differentiation between a conceptual model and its virtual representation, and its application to geometrical variations management is sketched. The paper provides a first theoretical and conceptual framework for the digital twin, which is to be enriched by the design and production engineering community in the future.The paper presents a comprehensive reference model for the digital twin in design and production engineering. The digital twin is a virtual representation of a physical product that enables the integration of design, manufacturing, and operational data throughout the product life-cycle. The authors propose a reference model based on the concept of Skin Model Shapes, which serves as a digital twin of the physical product. This model allows for the representation, implementation, and application of virtual product models in design and manufacturing. The model is designed to ensure scalability, interoperability, expansibility, and fidelity. The paper discusses the evolution of the digital twin concept, its current industrial understanding, and the challenges in its implementation. It also highlights the application of the reference model to geometrical variations management, which involves the differentiation between the real workpiece, its abstract model (Skin Model), and the virtual representation of this model. The paper concludes that a sound conceptual framework and a comprehensive reference model are essential for the successful implementation of the digital twin vision. The reference model enables the clear differentiation between a conceptual model and its virtual representation, and its application to geometrical variations management is sketched. The paper provides a first theoretical and conceptual framework for the digital twin, which is to be enriched by the design and production engineering community in the future.