Abstract: A method of modular design suitable for use, for example, in a CAD program is provided. In one aspect, the method is embodied in a CAD program specifically configured to support modular design, or more specifically, concurrent and consistent design of multiple installation assemblies of a product. Each installation assembly consists of a reference assembly and a Bill-of-Material (BOM) assembly. The BOM assembly contains component models that are the modular elements to be designed. The reference assembly contains referenced assemblies that are installed to make up the design environment for the component models in the BOM assembly. The BOM assembly in one installation assembly can be reused as a referenced assembly in the reference assembly of another installation assembly. Any changes made to the BOM assembly are automatically reflected in the reference assembly that contains the same BOM assembly as a referenced assembly.