We prove that every C^0 Jordan closed curve in the plane admits a nondegenerate inscribed square. We formulate the square condition as a zero problem for a distance-based map G. Using a ``two-sheet compactification'' and a boundary shape map g, we isolate collision/degeneracy/outer boundary components and remove them by a cut-out procedure. Route~B guarantees admissibility (the boundary does not hit 0), and admissibility is stable under small -perturbations via an explicit boundary margin. We then organize a collection of lemmas (Chapter~11) showing that key margins (self-separation, thickness, and cut-out room) persist under C^0\! C^1 approximation. For the reference curve S^1 we reduce the problem by quotienting out the S^1-action and define a reduced map H on a disk; a Jacobian computation yields ₂ (H) =1. A degree-bridge lemma gives ₂ (G) =₂ (H), and a domain-bridge proposition transfers this parity information to the cut-out domain W^ (0) for the full map F₀. Finally, homotopy invariance and the forcing theorem imply the existence of an interior zero, which corresponds to a nondegenerate inscribed square. This preprint is written for dissemination: we prioritize priority claims and a closed proof skeleton; local formalization can be added along the reference chain.
Yoshiki Ueoka (2025) studied this question.