### Video Transcript

A parallelogram whose what are
equal in length is called a rectangle.

We can recall that a rectangle
is a type of parallelogram with four congruent angles. The angles are all 90
degrees. We can sketch an example of a
rectangle and consider that in fact there is something which we know is equal in
length. The opposite sides in a
rectangle are equal in length. However, if we consider the
statement, we know that all parallelograms have opposite sides which are equal
in length. Therefore, we’re looking for
some additional property of rectangles which distinguish those from just any
parallelogram. That property comes from the
diagonals. The diagonals of a rectangle
are equal in length.

Note that in general the only
diagonal property of a parallelogram is that the diagonals of a parallelogram
bisect each other. Therefore, if we have got a
parallelogram and the diagonals are of equal length, then we know that it must
be a rectangle. We can therefore complete the
statement with the word diagonals. A parallelogram whose diagonals
are equal in length is called a rectangle.