A parallelogram is any quadrilateral, or four-sided shape, with opposite sides that are parallel and equal in length. Parallelograms that have four internal angles of 90 degrees per corner are rectangles or squares. One obtuse and one acute angle of a parallelogram add to 180 degrees and are known as supplementary angles. An acute angle subtracted from 360 degrees, known as a full rotation, equals the reflex angle, which is the exterior angle of the parallelogram.