Children with expressive language disorder have difficulty conveying or expressing information in speech, writing, sign language or gesture. (For preschool children, the difficulty expressing themselves in writing is not evident, as they have not started formal education.)
A rectangle is defined as a parallelogram (a quadrilateral with two sets of parallel sides) with the non-parallel sides joining perpendicularly (at 90Â°). A square is a rectangle with the additional constraint the all four sides be of equal length.
1 A square is a special case of a rectangle where all four sides are the same length. Adjust the rectangle above to create a square.  It is also a special case of a parallelogram but with extra limitation that the angles are fixed at 90Â°. See Parallelogram definition and adjust the parallelogram to create a rectangle.