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.
So a square IS still a rectangle (still has four right angles!). Both are also parallelograms. One way to see that this is useful, is to realize that any special properties from the larger class: e.g. diagonals bisect in a parallelogram) still apply to the special cases (e.g. a square or a rectangle).