A quadrilateral is a closed 2D polygon with four sides, four vertices, four angles, and two diagonals, where the sum of interior angles is always 360°. Examples include squares, rectangles, parallelograms, rhombuses, kites, and trapeziums. Quadrilaterals can be regular (all sides and angles equal) or irregular. A parallelogram has both pairs of opposite sides parallel, with special forms like the rectangle (right angles), rhombus (equal sides), and square (equal sides and right angles). A square is both a rectangle and a rhombus. Every parallelogram is also a trapezium, but not every trapezium is a parallelogram. Key properties vary by type — for example, parallelograms and rectangles have diagonals that bisect each other, while rhombuses have diagonals that intersect at right angles. Basic formulas include: Parallelogram – Area = base × height; Rectangle – Area = length × breadth; Rhombus – Area = ½ × (d₁ × d₂).