The present commentary discussed the emergence of symmetry as a pivotal behavior in linguistic, communicative and cognitive functions. Symmetry was defined as bidirectional stimulus-stimulus relationships (if AB then BA) in the stimulus equivalence which included transitivity and reversed transitivity (equivalence). First, the theoretical framework of symmetry and stimulus equivalence was presented from the point of conceptional, experimental and applied analysis of behavior. Second, the experimental researches by human and non-human were reviewed. The data suggested that humans in younger ages and with severe developmental disabilities established symmetry, though non-human animals did not show it in the usual method. In theoretical sense, symmetry and exclusion would be necessary for one-to-one correspondence between word and event as a basis of cognitive development. Third, I analyzed symmetry in linguistic and communicative function; comprehension⁄production, expression⁄reception, speaker-behavior⁄listener-behavior, initiating joint attention⁄responsive joint attention, imitation⁄counter imitation, and turn-taking (role-change) behavior. Fourth, symmetry could be applied as an important framework for speech and language therapy and learning and developmental intervention of reading and writing skills. Finally, issues of stimulus equivalence, verbal behavior and inner event were discussed in human development and cognitive⁄behavior therapy.