Three points are enough to define a circle as long as the three points does not lie in the same line (co-linear). This comes from the properties of the chords. With three points, three chords can be formed each having a perpendicular bisector. The intersection of these bisectors is the center of the circle defined by the given points. No parallel lines will only have a single intersection thus we are assured that only a single circle can be defined. Adjust the sliders below to change the position of the points. Observe the intersection of the bisectors and the circle formed.