Triangle Files

A triangle file (*.tri) is a text file containing triplets indicating the indicies of corners of triangles - used to define regions modelled by texture models, amongst other things.
You can generate and edit triangulation files using the am_tri_editor tool.

Here's a typical .tri file:

version: 1
n_triangles: 3
{
  { v1: 0 v2: 1 v3: 2 }
  { v1: 0 v2: 1 v3: 4 }
  { v1: 0 v2: 4 v3: 7 }
}