- The part of the plane enclosed by a simple closed figure is called a planar region.
- The measure of a planar region is called its area.
- Two figures are called congruent, if they have the same shape and the same size.
- If two plane figures are congruent, they have equal areas.
- Two plane figures having equal areas need not to be congruent.
Out of the two parallels one must be the line containing the common base.