The triangle occupies a surprisingly multivalent position within the depth-psychology corpus, functioning simultaneously as cosmological element, psychological symbol, geometric archetype, and epistemological figure. Plato’s Timaeus establishes the foundational valence: triangles are the irreducible elements of plane figures, and through them the four elements receive their formal constitution—fire, air, water, and earth each assigned a regular solid whose faces decompose into two fundamental triangular forms. This ontological grounding reverberates through later psychological thought. In Jungian and post-Jungian writing, the triangle accrues specifically trinitarian significance: Edinger reads the triangular image as representing the dynamic, threefold principle of thesis-antithesis-synthesis that breaks open static, circular completeness. The AA symbol—a triangle inscribed within a circle—links Wilson’s recovery theology to Jung’s mandala work and alchemical traditions of squaring the circle. Nichols, reading Tarot numerology, finds the triangle implicit in the six-pointed star as the union of heaven-pointing and earth-pointing apices. Descartes deploys the triangle repeatedly as the paradigm case for essence, existence, and clear-and-distinct perception. Merleau-Ponty uses it to contest formalist essentialism: the triangle’s truth is not analytic but phenomenological, dependent on its Gestalt as drawn. The term thus traverses cosmology, number mysticism, trinitarian theology, phenomenology, and symbolic amplification, making it one of the more richly layered geometric concepts in the tradition.