Two circles are said to be congruent if they have the same radius.

Congruent circles	Not congruent circles

To check whether two given circles C₁ and C₂ are congruent or not, trace C₁ and put it over C₂ such that, their centres coincide with each other. If the circle C₁exactly covers the circle C₂, we will see that they have the same radii. We then say that, the two circles are congruent to each other. We will write C₁ C₂.