edge3 versus edge2

[dealmeidavf]

This input

[Mesh]
  [1d]
   type = GeneratedMeshGenerator
   dim = 1
   nx = 4
   xmin = -5
   xmax = 5
   elem_type = edge3
  []
[]

[Variables]
  [u]
    order = second
    family = lagrange
  []
[]

will not run unless elem_type = edge3. The error is:

ERROR: Bad ElemType = EDGE2 for SECOND order approximation!

What is the difference between edge2 and edge3.
Thanks.

[GiudGiud]

For second order variables you need a second order mesh.
Specify second_order = true in the [Mesh] block directly then you dont need to specify the elem_type for it to work I think
The difference is the number of dofs on the edge.

[dealmeidavf]

I tried but still got

ERROR: Bad ElemType = EDGE2 for SECOND order approximation!

It is ok. I understand the EDGE3 for quadratic Lagrange polynomials in 1D. The "edge" naming did not connect to 1D at first.
Thanks.