If you were to remove the right angled triangle, starting from the base on one side of a parallelogram, you would find that it would fit exactly onto the other side of the parallelogram to form a rectangle.

So the area of a parallelogram can be measured by multiplying its length by its perpendicular height, i.e. its height measured at a 90° (right) angle from the base.

This parallelogram measures 5 cm along the base and has a height of 2 cm, so the area is:

10 x 2 = 20 cm²