According to the Monterey Institute, quadrilaterals with four congruent sides are called regular quadrilaterals and include squares and rhombuses. A quadrilateral is a polygon with four sides, and the term "regular" means that all sides are congruent, or the same size.
As a polygon, a quadrilateral has a two-dimensional shape with four sides and four angles. Examples of quadrilaterals are rectangles, parallelograms, squares and rhombuses. Only squares and rhombuses are considered regular quadrilaterals because their sides are all the same length. A square is a quadrilateral with four congruent sides, with the opposite sides parallel, and four right angles. A rhombus has four congruent sides that do not form right angles.