  Topological phase transitions, which have fascinated generations of
physicists, are always demarcated by gap closures. In this work, we propose
very simple 2D photonic crystal lattices with gap closure points, i.e. band
degeneracies protected by nonsymmorphic symmetry. Our photonic structures are
relatively easy to fabricate, consisting of two inequivalent dielectric
cylinders per unit cell. Along high symmetry directions, they exhibit line
degeneracies protected by glide reflection symmetry, which we explicitly
demonstrate for $pg,pmg,pgg$ and $p4g$ nonsymmorphic groups. In the presence of
time reversal symmetry, they also exhibit point degeneracies (Dirac points)
protected by a $Z_2$ topological number associated with crystalline symmetry.
Strikingly, the robust protection of $pg$-symmetry allows a Lifshitz transition
to a type II Dirac cone across a wide range of experimentally accessible
parameters, thus providing a convenient route for realizing anomalous
refraction. Further potential applications include a stoplight device based on
electrically induced strain that dynamically switches the lattice symmetry from
$pgg$ to the higher $p4g$ symmetry. This controls the coalescence of Dirac
points and hence the group velocity within the crystal.
