<Tr> <Td> 3.3. 3 (Vertex figure) </Td> <Td> Self - dual (dual polyhedron) </Td> </Tr> <Tr> <Td_colspan="2"> Net </Td> </Tr> <P> In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners . The tetrahedron is the simplest of all the ordinary convex polyhedra and the only one that has fewer than 5 faces . </P> <P> The tetrahedron is the three - dimensional case of the more general concept of a Euclidean simplex, and may thus also be called a 3 - simplex . </P>

What is the shape of each face of a tetrahedron