- A simple closed curve made up of only line segments is called a polygon.
- A diagonal of a polygon is a line segment connecting two nonconsecutive vertices.
- A convex polygon is a polygon in which no portion of its any diagonal is in its exterior.
- A quadrilateral is a polygon having only four sides.
- A regular polygon is a polygon whose all sides are equal and also all angles are equal.
- The sum of interior angles of a polygon of n sides is (n-2) straight angles.
- The sum of interior angles of a quadrilateral is 360°.
- The sum of exterior angles, taken in an order, of a polygon is 360°.
- Trapezium is a quadrilateral in which a pair of opposite sides is parallel.
- Kite is a quadrilateral which has two pairs of equal consecutive sides.
- A parallelogram is a quadrilateral in which each pair of opposite sides is parallel.
- A rhombus is a parallelogram in which adjacent sides are equal.
- A rectangle is a parallelogram in which one angle is of 900.
- A square is a parallelogram in which adjacent sides are equal and one angle is of 900.
- In a parallelogram, opposite sides are equal, opposite angles are equal and diagonals bisect each other.
- In a rhombus diagonals intersect at right angles.
- In a rectangle diagonals are equal.
- Five measurements can determine a quadrilateral uniquely.
- A quadrilateral can be constructed uniquely if the lengths of its four sides and a diagonal are given.
- A quadrilateral can be constructed uniquely if the lengths of its three sides and two diagonals are given.
- A quadrilateral can be constructed uniquely if its two adjacent sides and three angles are given.
- A quadrilateral can be constructed uniquely if its three sides and two included angles are given.
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