The Formal Rules Hidden in The Plans of Congregational Mosques as Revealed Congregational mosques are among the most important products of Islamic architecture. They gained their importance from their religious value derived from their devotional function. However, they were characterized by multiple functions, as they were centers of education, practicing governance, political decision-making, launching of armies and finally, practicing various sports. Congregational mosques were always among the first facilities to be created in Islamic cities, whether those cities that existed before Islam or new cities. Over the successive centuries, many congregational mosques were built with different formal styles and numerous design characteristics. Congregational mosques have acquired a formal and visual identity that distinguish them from other buildings within Islamic Architecture, and other Architectures. The design and construction of congregational mosques has been a process that has continued over many centuries up to the present time, which witnessed the production of new mosques, whether congregational or small ones. A review of the large number of mosques that were produced after the mid-twentieth century and the beginning of the twenty-first century reveals multiple and diverse intellectual trends and design practices with varying degrees of approach or distance from the identity that Islamic mosques have acquired throughout their history. The current research raises the importance of preserving the original identity of Islamic mosques and that new products are not far from the original designs that are associated with the Islamic faith in which they are manifested and associated with their original values. In this context, the current research intends to approach the original Islamic Mosques through the concept of Shape Grammers, which has gained exceptional importance for architecture, as it is adopted for purposes related to descriptions, analyses and objective evaluations of various architectural products. It can also be used for purposes related to deriving and generating new designs, ensuring their richness and diversity. Above all, revealing the implicit design process by which the architectural product was created, no matter how old it is, and for which no data is available regarding the design process by which it was produced. The research intends to conduct a study in which the congregational mosques are a context for applying the methodology of Shape Grammers. It aims to identify and describe, objectively, the formal rules for the various types of congregational mosques first, and to reapply the extracted rules to generate and derive new innovative solutions that belong to the same formal language and are not far from their original identity. To achieve the above-mentioned goal, several methodological tasks were implemented within four stages: First: Selecting the case studies as thirty-two congregational mosques were selected with their different formal styles. Second: Analysis of the case studies using Shape Grammers methodology. Third: Synthesis of the formal rules that implied the creation of the congregational mosques that were analyzed in the previous step. Fourth: Presenting an attempt to generate new mosques that respect the formal rules that produced the original congregational mosques. The conclusions supported the main research hypothesis, which stated that congregational mosques, from the perspective of Shape Grammers, are nothing but the results of applying formal rules of a specific nature on an initial form. Accordingly, congregational mosques can be considered as different formulas for applying specific formal rules with a specific number and a specific nature. New mosques were also generated within the framework of the same formal rules that gave the old congregational mosques their identity and original design features.