Boundary markers are tags used mainly to identify which output vertices and edges are associated with which PSLG segment, and to identify which vertices and edges occur on a boundary of the triangulation. A common use is to determine where boundary conditions should be applied to a finite element mesh. You can prevent boundary markers from being written into files produced by Triangle by using the -B switch.

The boundary marker associated with each segment in an output .poly file and each edge in an output .edge file is chosen as follows:

- If an output edge is part or all of a PSLG segment with a nonzero boundary marker, then the edge is assigned the same marker as the segment.
- Otherwise, if the edge occurs on a boundary of the triangulation (including boundaries of holes), then the edge is assigned the marker one (1).
- Otherwise, the edge is assigned the marker zero (0).

- If a vertex is assigned a nonzero boundary marker in the input file, then it is assigned the same marker in the output .node file.
- Otherwise, if the vertex lies on a PSLG segment (including the segment's endpoints) with a nonzero boundary marker, then the vertex is assigned the same marker. If the vertex lies on several such segments, one of the markers is chosen arbitrarily.
- Otherwise, if the vertex occurs on a boundary of the triangulation, then the vertex is assigned the marker one (1).
- Otherwise, the vertex is assigned the marker zero (0).

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