The abstract, general-relativistic form of the Weyl equation can be understood as follows: given a pseudo-Riemannian manifold M , one constructs a fiber bundle above it, with the spin group as the fiber. The spin group S p i n ( p , q ) is a double cover of the special orthogonal group S O ( p , q ) , and so one can identify the spin group fiber-wise with the frame bundle over M . When this is done, the resulting structure is called a spin structure.