  A general and beautiful picture for the realization of topological insulators
is that the mass term of the Dirac model has a nodal surface wrapping one Dirac
point. We show that this geometric picture based on Dirac points can be
generalized to extended band degeneracies with nontrivial topological charges.
As the nontrivial topological charges force the extended band degeneracies to
be created or annihilated in pairs, when the nodal surface of mass wraps one
such extended band degeneracy, the resulting gapped phase must be topologically
nontrivial since it cannot adiabatically be deformed into a topologically
trivial atomic insulator without closing the energy gap. We use nodal lines
carrying a nontrivial $Z_{2}$ monopole charge in three dimensions to illustrate
the physics. Notably, because the wrapping surfaces for an extended band
degeneracy are diverse, we find that this generalization can bring topological
insulators with unconventional pattern of boundary states.
