The quaternio stands as one of the most architecturally ambitious concepts in Jung’s depth psychology, functioning simultaneously as a structural schema for the psyche, a cosmological organizing principle, and a hermeneutic tool for reading alchemical, Gnostic, and mythological material. Across the corpus, the term designates a fourfold arrangement of psychic elements whose internal tensions generate transformation: the Anthropos Quaternio, the Moses Quaternio, the Paradise Quaternio, the Shadow Quaternio, and the Lapis Quaternio form a cascading series in Aion that maps the full vertical extent of psychic life from spirit to chthonic matter. Jung insists that the quaternio is not merely a number-mystical curiosity but ‘an organizing schema par excellence,’ analogous to the crossed threads of a telescope — a coordinate system imposed by the psyche upon the chaos of experience. The persistent tension between three and four, elaborated with particular acuity by von Franz and Pauli, animates the quaternio’s deepest significance: four achieves what three cannot, namely the inclusion of matter, the feminine, and the inferior function, completing the self’s wholeness. Edinger and Stein extend this to clinical and pedagogical registers, while Pauli anchors the fourfold schema in the history of natural philosophy from Fludd to Kepler. The quaternio is thus the psyche’s own grammar of totality.