The author considers developmental and transferential implications of the lateral dimension of psychic life, which entails the challenge to forge a unique and valued identity in a world of one’s equals, beginning with siblings. The lateral dimension intersects with the vertical dimension of hierarchical relationships, notably those between child and parent. Consideration of the lateral dimension and its particular developmental challenge promises to expand our understanding of the processes of identity development and of transferences involving both siblings and parents. Moreover, conceiving of psychic life as comprising two dimensions may help to contextualize existing psychoanalytic understanding of sibling-related processes in light of fundamental tenets of psychoanalytic theory. A de…