Question: why are the diagonals of a parallelogram not perpendicular?
Answer: The diagonals of parallelogram are not perpendicular to each other is it a rhombus? ... rhombus is constructed ! when length l is reflected, its length does not change, hence only a rhombus can be constructed this way and not parallelogram. the angle between diagonals = angle between x & y axis = 90 degrees.