# Question about face numbering in hexahedron cells

**URL:** https://discourse.vtk.org/t/question-about-face-numbering-in-hexahedron-cells/3286
**Category:** Support
**Created:** [May 12, 2020, 5:49pm UTC](https://discourse.vtk.org/t/question-about-face-numbering-in-hexahedron-cells/3286 "2020-05-12T17:49:20Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![lijas](https://discourse.vtk.org/user_avatar/discourse.vtk.org/lijas/32/1743_2.png) [@lijas](https://discourse.vtk.org/u/lijas)
#### Post date: [May 12, 2020, 5:49pm UTC](https://discourse.vtk.org/t/question-about-face-numbering-in-hexahedron-cells/3286/1 "2020-05-12T17:49:20Z")

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For the cell [vtkTriQuadraticHexahedron](https://vtk.org/doc/nightly/html/classvtkTriQuadraticHexahedron.html), the face numbering is defined here as

> The mid-surface nodes lies on the faces defined by (first edge nodes id’s, than mid-edge nodes id’s): (0,1,5,4;8,17,12,16), (1,2,6,5;9,18,13,17), (2,3,7,6,10,19,14,18), (3,0,4,7;11,16,15,19), (0,1,2,3;8,9,10,11), (4,5,6,7;12,13,14,15)

```auto
    * top
    * 7--14--6
    * | |
    * 15 25 13
    * | |
    * 4--12--5
    *
    * middle
    * 19--23--18
    * | |
    * 20 26 21
    * | |
    * 16--22--17
    *
    * bottom
    * 3--10--2
    * | |
    * 11 24 9
    * | |
    * 0-- 8--1

```

I interpret this as follows:

> front, right, back, left, bottom, top

But for [higher order elements](https://blog.kitware.com/wp-content/uploads/2020/03/Implementation-of-rational-Be%CC%81zier-cells-into-VTK-Report.pdf), explained in this pdf, the order in figure 8 seems to be

> left,right,front,back,bottom,top.

I am guess that vtk use the same face order for all cells, so which one is wrong? or am I misinterpreting something?

---

<div class="post-metadata">

### Author: ![lijas](https://discourse.vtk.org/user_avatar/discourse.vtk.org/lijas/32/1743_2.png) [@lijas](https://discourse.vtk.org/u/lijas)
#### Post date: [May 14, 2020, 9:26am UTC](https://discourse.vtk.org/t/question-about-face-numbering-in-hexahedron-cells/3286/2 "2020-05-14T09:26:29Z")

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Does anyone know if it is different face ordering for vtkTriQuadraticHexahedron and “higher order lagrange cells”, or if it is wrong in the documentation?