Cell Types

meshio++ uses its own canonical type names. Every format reader maps native element names to these; every writer maps them back.

Node ordering follows the VTK convention where available. The figures below are generated from the code's own topology tables (_convert_cells._ELEVATE for mid-edge nodes, _skin._CELL_FACES and _refine_templates.QUAD_FACES for face and body centres), so they cannot drift from what the readers, writers and operations actually assume; corner nodes are drawn solid, mid-edge nodes blue, face centres orange and the body centre violet. One convention is worth stating: pyramid14 is pyramid13 plus a base-centre node 13, the order every consumer in the core (detail/cell_faces.hpp, the CGNS table) uses; the gmsh reader/writer permutes both pyramid14 and wedge18 (gmsh's own edge-lexicographic types 14 and 13) to and from this convention, derived from gmsh's own edge/face tables and verified geometrically (every mid-edge/face-centre node lands at the exact midpoint/centroid of the corners it should).

0-D

Type	Nodes
vertex	1

1-D (line elements)

Type	Nodes	Description
line	2	Linear line
line3	3	Quadratic line
line4	4	Cubic line
line5	5	Quartic line
line6	6
line7	7
line8	8
line9	9
line10	10
line11	11

2-D surface elements

Triangles

Type	Nodes
triangle	3
triangle6	6
triangle7	7 (triangle6 plus a centre node, VTK 34; Code_Aster TRIA7, MED TR7)
triangle10	10
triangle15	15
triangle21	21
triangle28	28
triangle36	36
triangle45	45
triangle55	55
triangle66	66

Quadrilaterals

Type	Nodes
quad	4
quad8	8
quad9	9
quad16	16
quad25	25
quad36	36
quad49	49
quad64	64
quad81	81
quad100	100
quad121	121

Arbitrary polygons

Type	Nodes
polygon	variable

For polygon cells, all cells within a CellBlock must have the same number of nodes (the array shape determines this). Multiple CellBlock entries with type polygon but different node counts are permitted.

3-D volume elements

Tetrahedra

Type	Nodes
tetra	4
tetra10	10
tetra20	20
tetra35	35
tetra56	56
tetra84	84
tetra120	120
tetra165	165
tetra220	220
tetra286	286

Hexahedra

Type	Nodes
hexahedron	8
hexahedron20	20
hexahedron24	24
hexahedron27	27
hexahedron64	64
hexahedron125	125
hexahedron216	216
hexahedron343	343
hexahedron512	512
hexahedron729	729
hexahedron1000	1000
hexahedron1331	1331

Wedges (prisms)

Type	Nodes
wedge	6
wedge15	15
wedge18	18
wedge40	40
wedge75	75
wedge126	126
wedge196	196
wedge288	288
wedge405	405
wedge550	550

Pyramids

Type	Nodes
pyramid	5
pyramid13	13
pyramid14	14

Arbitrary polyhedra

Type	Description
polyhedron4	Tetrahedron as polyhedron
polyhedron5	Pyramid as polyhedron
polyhedronnN	N-faced polyhedron

Polyhedron cells store their face connectivity as a list of lists (ragged), not a rectangular numpy array. The type name encodes the number of faces: polyhedron4 has 4 faces.

VTK Lagrange types

Higher-order VTK Lagrange elements (arbitrary polynomial order, controlled at runtime):

Type
VTK_LAGRANGE_CURVE
VTK_LAGRANGE_TRIANGLE
VTK_LAGRANGE_QUADRILATERAL
VTK_LAGRANGE_TETRAHEDRON
VTK_LAGRANGE_HEXAHEDRON
VTK_LAGRANGE_WEDGE
VTK_LAGRANGE_PYRAMID

These are read and written by the VTK/VTU readers/writers, and read and written by the MFEM reader and writer for meshes and fields of order 3 and up (v16.11.0; VTK_LAGRANGE_PYRAMID excepted). They are ragged blocks: the node count follows from the order, which is not stored in the type.

VTK Bézier types

VTK's rational Bézier elements (since v16.40.0), the same seven shapes:

Type
VTK_BEZIER_CURVE
VTK_BEZIER_TRIANGLE
VTK_BEZIER_QUADRILATERAL
VTK_BEZIER_TETRAHEDRON
VTK_BEZIER_HEXAHEDRON
VTK_BEZIER_WEDGE
VTK_BEZIER_PYRAMID

A Bézier cell lists its control points in the node order of the Lagrange cell of the same shape and degree, so it is a ragged block in the same way; but only its corners lie on the geometry, so operations do not treat it as a Lagrange cell, and drawing it (render) goes through its corners. VTK/VTU, VTKHDF and the legacy VTK format read and write them; a VTU's rational weights and per-cell degrees travel as the RationalWeights point data and the HigherOrderDegrees cell data (VTU).