A spin structure is a consistent choice of lifting oriented orthonormal frame changes to their spin double cover, permitting globally defined spinor fields. For a spatial circle in a worldsheet, the two choices give periodic and antiperiodic fermions, hence the Ramond sector and Neveu–Schwarz sector. Local field equations do not choose the global spin structure.
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Spin structure is a concept from topology and theoretical physics that arises in the context of manifold theory, particularly in relation to spin manifolds. In mathematics, a spin structure is typically defined on a manifold that enables the definition of spinors, which are mathematical objects that generalize the notion of complex numbers and vectors.