A regular quadrilateral is also called a square. Regular polygons are two-dimensional shapes in which all sides are of equal length and all angles are of equal breadth. A quadrilateral is any four-sided polygon. A square has four sides and four angles, all of equal size.
The interior angles of a square always add up to 360 degrees. Each individual angle contains 90 degrees. Other kinds of quadrilaterals may have equal sides or equal angles but not both. For example, every angle in a rectangle is of equal breadth, each angle containing 90 degrees. However, the sides differ in length. In a rhombus, all of the sides are equal, but the breadth of the angles differ.