Another type of polygon has four sides and is called a quadrilateral. The sum of its four angles is always 360°. The sum of the lengths of its sides is called the perimeter.
A parallelogram has four sides, of which the opposite ones are parallel.
A square has four equal sides, and each of its four angles is 90°.
A rectangle is a parallelogram with four angles of 90° each.
A rhombus has four equal sides. Two of its angles are obtuse, that is greater than 90°.
A trapezoid has only two parallel sides.
A trapezoid has no parallel sides.
Regular and irregular polygons (Polygon means polygon) can have more than three or four sides. For example, a pentagon has five sides, a hexagon six, a heptagon seven, an octagon eight and a dode-kagon twelve sides.