A quadrilateral is a closed figure bounded by four line segments.

A quadrilateral ABCD has
- four vertices (A,B,C and D)
- four sides (AB, BC, CD and DA)
- four angles (∠A, ∠B, ∠C and ∠D)
- two diagonals (AC and BD)

### Adjacent sides

Two sides of a quadrilateral having a common end-point are called adjacent or consecutive sides. There are four pairs of it.

In the quadrilateral (AB,BC), (BC,CD), (CD,DA) and (DA,AB) are such sides.

### Opposite sides

Two sides of a quadrilateral having no common end-point are called opposite sides. There are two pairs of it.

In the quadrilateral (AB,CD) and (BC,DA) are such sides.

### Adjacent angles

Two angles of a quadrilateral having a common side are called adjacent or consecutive angles. There are four pairs of it.

In the quadrilateral (∠A,∠B), (∠B,∠C), (∠C,∠D) and (∠D,∠A) are such angles.

### Opposite angles

Two angles of a quadrilateral having no common side are called opposite sides. There are two pairs of it.

In the quadrilateral (∠A,∠C) and (∠B,∠D) are such sides.

## Parallelogram

A quadrilateral in which opposite sides are parallel is called a parallelogram.

ABCD is a parallelogram. AB||CD and BC||DA.

## Properties of a parallelogram

- opposite sides are equal
- opposite angles are equal
- diagonals bisect the parallelogram

## Rectangle

A parallelogram in which all the angles are 90° is called a rectangle.

ABCD is a rectangle.

## Properties of a rectangle

- opposite sides are equal
- each angle measures 90°
- diagonals are equal

## Square

A rectangle in which all sides are equal is called a square.

ABCD is a square.

## Properties of a square

- all sides are equal
- each angle measures 90°
- diagonals are equal
- diagonals intersect at 90°

## Rhombus

A parallelogram in which all sides are equal is called a rhombus.

Ab=CD=DA=AB

ABCD is a rhombus.

## Properties of a rhombus

- opposite sides are parallel
- all sides are equal
- diagonals bisect each other at right angles

## Trapezium

A quadrilateral in which two opposite sides are parallel is called a trapezium.

ABCD is a Trapezium.

AB||DC.

If the non parallel sides are equal then the trapezium is called isosceles trapezium.