The variety of physical factors and chemical agents used makes it necessary to study the general laws of the manifestation of synergy. It is well known that the maximum effect of the combined action of two agents is achieved with their simultaneous action. The classics of the theory of chemical equilibrium have already in the last century found out for themselves that the formation of mixed complexes is an indispensable condition for the appearance of synergy. However, since then, the attention of researchers has been focused on the external conditions for the manifestation of synergistic effects, neglecting this necessary condition. In this paper the common feature for all synergistic chemical processes has been elucidated. Frequently this common feature does not come to light, being ―hidden‖ and thus the phenomenon of synergism is explained in various ways, depending on investigated chemical processes and their peculiarities, namely the chemical structures of the involved reagents, their mechanism, kinetics, energetics, etc. The authors have proved for all chemical processes used in various physicochemical methods of analysis and research and different fields of applications, that the common feature of the phenomenon of synergism is the formation of a mixed complex. The paper intends to focus attention, describe and characterize this common feature as a suitable tool for search and design of new synergistic effects with required properties for different chemical processes. The authors have formulated the following rule: the formation of mixed complexes is a necessary, but not sufficient condition to explain the occurrence of the chemical synergism. To prove the synergistic effect, it is necessary to measure a certain physicochemical or physical quantity, which proves the superiority of the non-additivity phenomenon over a simple summation of the properties of individual components. In other words, the theory of chemical equilibrium cannot serve as a basis for predicting synergy, but it can be post-proof of this phenomenon. The knowledge and the application of the ideas and general patterns of synergy described can be useful for specialists using the simultaneous action of various agents to optimize combined technological schemes. It was also concluded that the overall universal pattern of the manifestation of synergy is its dependence on the intensity of the agents used. The existence of universal patterns of synergy indicates the need to develop a new paradigm and theoretical concept of chemical synergy, which should take into account the identified patterns. A prior knowledge of this common feature gives the possibility of a deeper understanding of this phenomenon and of target search or prediction of required or pre-planned synergistic effects. Subsequent research on chemical synergism in order to predict this phenomenon should focus on finding sufficient conditions for synergistic systems, in which the formation of the mixed compound is well known and their comparison with other non-synergistic analogous systems, in which a mixed compound is formed. To explain the lack of synergism in the systems where the formation of the mixed compound takes place, additional knowledge is needed from other sciences, which studies the structure and reactivity of chemicals.