Ring of band crossings in the BZ, with flat drumhead states inside the projected nodal ring
Nodal line semimetals have band crossings forming a closed ring (or line) in the Brillouin zone, protected by PT symmetry or mirror symmetry. Spin-orbit coupling (SOC) gaps the nodal line into a pair of Weyl points. The surface projection of the ring interior hosts flat "drumhead" surface states — analogous to the flat edge states of 2D systems, but spanning a 2D patch.