Spindle Tetrahedron

A spindle tetrahedron is a long, thin tetrahedron whose four vertices lie close to four distinct positions on a line (Cheng et al. 2012, p. 25; Hannukainen et al. 2017, p. 3).

As it approaches this degenerate configuration, its volume tends to zero without requiring any pair of vertices to coalesce. This distinguishes it from a splinter tetrahedron, which has two close pairs. Both names belong to qualitative tetrahedral mesh classifications.