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.