When you work with circles, there are three straight-line components that you need to be able to identify: radii, chords, and diameters.

  • Radius: A circle’s radius — the distance from its center to a point on the circle — tells you the circle’s size. In addition to being a measure of distance, a radius is also a segment that goes from a circle’s center to a point on the circle.

  • Chord: A segment that connects two points on a circle is called a chord.

  • Diameter: A chord that passes through a circle’s center is a diameter of the circle. A circle’s diameter is twice as long as its radius.


The above figure shows circle O.