Modal Triplet Theory (MTT) derives spacetime geometry as an effective description emerging from coherent sectors of an underlying modal dynamics. While derivational work shows how a Lorentzian spacetime with (3+1) signature can arise, a separate question remains: why alternative spacetime signatures do not yield stable, admissible physical descriptions under the same framework. This paper addresses that question. Treating spacetime signature as an emergent property of coherent observables rather than a fundamental assumption, it analyzes candidate signatures—Euclidean, (2+2), and higher-dimensional Lorentzian—and shows that they fail to satisfy the combined requirements of stability, coherence invariance, controlled truncation, and admissibility. The (3+1) Lorentzian signature is thereby selected as the minimal configuration compatible with the canonical assumptions of MTT. This selection is conditional rather than axiomatic: alternative signatures are mathematically conceivable but excluded by structural failure modes rather than metaphysical necessity. The paper complements spacetime derivation results by making the exclusion logic explicit.
Peter Nero (Fri,) studied this question.