By definition, a PSLG is just a list of points and segments. A .poly file can also contain information about holes and concavities, as well as regional attributes and constraints on the area of triangles. The first section lists all the points, and is identical to the format of .node files. This has the advantage that a point set may easily be triangulated with or without segments. The second section lists the segments .
Segments are edges whose presence in the triangulation is enforced.
Each segment is specified by listing the indices of its two endpoints. This means that you must include its endpoints in the point list. Each segment, like each point, may have a boundary marker . Holes are specified by identifying a point inside each hole. If the two triangles abutting a segment are eaten, the segment itself is also eaten.