(a) Circumcircle
A circle passing through the vertices of any triangle (polygon) is called circumcircle. The centre of this circle is called the circumcentre and its radius is called the circumradius. The radius of circumcircle is denoted by R.
(b) Incircle
A circle drawn inside and touching the sides of a triangle (polygon) is called incircle or inscribed circle. It is the largest circle contained in the triangle. The centre of this circle is called the incentre usually denoted by I and its radius is called the inradius denoted by r.
(c) Escribed Circle
A circle touching one of the sides of a triangle externally and the extensions of its two other sides internally, is called an escribed circle (e- circle or ex- circle). The centre of this circle is called the e- centre and its radius is called the e- radius.