Circles are geometric figures whose points all lie the same distance from a given point, the circle's centre. In brief, a circle is a closed plane curve consisting of all points at a given distance from a fixed point within it, which is called the centre of the circle. The centre of a circle always lies in the interior of the circle. The distance between any points of the circumference of the circle and the centre of the circle is called the radius. Also, the longest chord of a circle is its diameter. Chapter 16 mainly deals with basic definitions, arcs of a circle, chord and segment of a circle, congruence of circles and arcs, some results on equal chords, arcs and angles subtended by them and cyclic quadrilateral.