Although a lot of research has been carried out on the technical architecture of software systems, the domain of Functional Architecture in the software product industry lacks a formalization of the related concepts and practices. Functional Architecture Modeling is essential for identifying the functionalities of the software product and translating them into modules, which interact with each other or with third party products. Furthermore, the functional architecture serves as a base for mapping the requirements and planning the product releases. Some software vendors intuitively tend to design the functional architecture of their products, but this practice has been unstructured so far and realized only in specific business domains. In this paper, we present the Functional Architecture Diagrams, a powerful modeling tool for the functional architecture of software products, which comprises: a modular decomposition of the product functionality; a simple notation for easy comprehension...