Rectangle. A quadrilateral whose four angles are equal, that is, whose 4 angles are equal to .

Any rectangle is a parallelogram.
This follows from angles and adding up .

Since parallelograms have their opposite sides equal, so do rectangles. In the picture, and .

Rectangles are the only parallelograms to be also cyclic (since opposite angles add up .

Notice that every square is also a rectangle, but there are rectangles that are not squares

Rectangles have their two diagonals equal (since triangles and are congruent), A nice result following from this is that the quadrilateral obtained by joining the midpoints of the sides is a rhombus.

Since joins midpoints of sides in triangle , we have . Similarly we have , and and thus the sides of quadrilateral are all equal, in other words, is a rhombus.