Normal surface
= Normal surface
Relative to a <triangulation>, a <normal surface> meets every <tetrahedron> in disjoint normal <disks>: four types of triangles cutting off vertices and three types of quadrilaterals separating opposite pairs of edges. A normal <disk> meets any edge at most once; its face arcs join distinct edges. Two different quadrilateral types cannot coexist in a <tetrahedron>.