Abstract—At Mjølner, we apply a method for Physical-Digital Product Development, which we call Multi-Source Design (MSD). This builds on several existing well-known best-practices, which we have combined and applied on a wider scope encompassing adjacent disciplines to software development such as hardware and mechanical development. We see the MSD method as a contemporary approach to physical-digital product development by integrating agility, modularity, and decoupling. MSD adapts principles from software development to create resilient and adaptable physical-digital products, addressing vulnerabilities such as supply-chain disruptions, high maintenance costs, and inflexible product architectures. This paper describes the motivation behind MSD, introduces the method itself, describes experiences from industries such as automotive or electronics and considers the benefits of the method.