Developmental psychologists have a long history of using triangle metaphors to conceptualise the social constitution of psychological development. In this paper, we present a genealogy of triadic theories, to clarify their origins, distinctions between them, and to identify key themes for theoretical development. The analysis identifies three core triangle models in the developmental literature. Each theory relies on some combination of the terms subject, object, other and sign, and they can be distinguished by the core psychological dynamic which they entail. We distinguish an emotional triangle rooted in psycho-analysis, a mediational triangle rooted in the work of Vygotsky, and a sociocognitive triangle originating with Piaget. Despite their differences, the analysis reveals a common theme of the transformation from external mediation to internal mediation. Contemporary research and possible future directions are discussed in the light of the theoretical distinctions that our genealogy has revealed.