Tetrahedron orientation
This refers to the order of vertices within a face. Typically in 3D graphics, faces are wound counterclockwise, so that if you're looking from outside of the tetrahedron/mesh, all the front faces will have their vertices in counterclockwise order. (See here and here for more explanation.) This is a &quot;right-hand rule&quot; in that if the fingers of your right hand curl in the winding direction of the vertices, then your thumb points toward the outside of the mesh.