A topic related to the nature of parallelogram (1) in chapter 4.4. 1 of the first volume of eighth grade mathematics of Beijing Normal University. .....
Vertical, because EB=BC can be proved, so we get the angle EBC=CBE, and there are four flat ABCDs, so we can prove that the angle BEC=DCE, so the angle DCE=BCE, the same angle ADF=CDF, the angle ADC+BCD= 180, so the angle CDF+DCE=90, which is a triangle.