What are perpendicular and parallel lines on a polygon?
A polygon need not have ANY perpendicular or parallel lines. For
example, consider an equilateral triangle.
It can happen that two sides of a polygon, extended if
necessary, meet at a point where they form a 90 degree angle. Those
two lines are perpendicular.
There may be pairs of lines such that, no matter how far you
extend them in either direction, they will never meet. Such lines
are parallel.
A triangle cannot have parallel lines but it can have
perpendicular lines. Any polygon of 4 or more sides can have sides
that are perpendicular or parallel (or some of each).