  Classification of topological insulators and superconductors is manifested in
terms of spin cobordism groups for lower dimensions. It is discussed that the
periodic table of topological insulators is a result of the possible choices of
spin structures on Brillouin zones of relevant topological materials. This
framework is extended to the case of Weyl semimetals. It is shown that the
classification of Weyl semimetals can be managed in terms of spin$^c$ cobordism
groups via the extension of spin structures to spin$^c$ structures. Topological
invariants of Weyl semimetals are connected to the topological invariants of
spin$^c$ cobordism groups and Fermi arcs of Weyl semimetals are interpreted in
terms of the choices of spin$^c$ structures.
