Design of complex configurable products is a hard task. Testing of all variants is often impossible because the number of variants is orders of magnitude larger than the total life cycle production volume. Modular design is advocated as solution but cannot guarantee that if one variant works OK another will not cause errors. This paper proposes, based on a similar method for modular database system design, a theory for modular design of mechanical systems that poses the concept of module independence, to enable design and test of module families as a separate unit while offering formal conditions to assure that no variant of the module family will cause failures when integrated in the end-product. This allows system verification and testing to be done per module family with no need to test individual end product variants. The method is illustrated with a simple example of a mechanical design, but not yet applied in practice.