The word ‘quad’ means four and the word lateral means sides. Thus, a plane figure bounded by four line segments AB, BC, CD and DA is called a quadrilateral and is written as quad. ABCD. Vertices Sides
i) A i) AB
ii) B ii) BC
iii) C iii) CD
iv) D iv) DA

Angles Diagonals

i) ∠A i) AC
ii) ∠B ii) BD
iii) ∠C
iv) ∠D

On the basis of diagonals, there are two types of quadrilaterals.

1) Convex
2) Concave

In
Convex quadrilateral , the diagonals intersect in the interior region and in Concave quadrilateral , one of the diagonal is in the exterior region .  1) Adjacent sides : Two sides of a quadrilateral are called adjacent sides, if they have a common end point.

According to the given diagram, the adjacent sides are

i) AB, BC
ii) BC, CD
iii) CD, DA
iv) DA, AB

2) Opposite sides : Two sides of a quadrilateral are called its opposite sides, if they do not have a common end point.

i) AB and CD
are two pairs of opposite sides of the quadrilateral ABCD.

Diagonals : It is formed by the segment joined by the opposite vertices.

AC and BD are diagonals.

Adjacent angles : Two angles of a quadrilateral are called adjacent angles, if they have a common side as an arm.

i) ∠A , ∠B
ii) ∠B, ∠C
iii) ∠C , ∠D
iv) ∠D , ∠ A

Opposite angles : Two angles of a quadrilateral are called opposite angles which are not adjacent angles.

i) ∠A , ∠C
ii) ∠B , ∠D are two pairs of opposite angles of the quadrilateral ABCD.