A polygon with four sides.
Quadrilaterals can be grouped into various classes based on their properties, as shown in the diagram below. For example, the class of parallelograms groups all the convex quadrilaterals, which, in addition to having at least one pair of parallel sides, also belong to the more general class of trapezoids. The class of parallelograms has an added property, in that these figures have two pairs of parallel sides.
In the examples below, the congruent sides are marked with small hash marks, and the congruent angles are marked with small arcs if they are not right angles. They are marked with small squares if they are right angles.