You show the diameter and radius of a circle by drawing segments from a point on the circle either to or through the center of the circle. But two other straight figures have a place on a circle. One of these figures is called a chord, and the other is a tangent:
Chords: A chord of a circle is a segment that you draw from one point on the circle to another point on the circle. A chord always stays inside the circle. The largest chord possible is the diameter — you can't get any longer than that segment.
Tangent: A tangent to a circle is a line, ray, or segment that touches the outside of the circle in exactly one point. It never crosses into the circle. A tangent can't be a chord, because a chord touches a circle in two points, crossing through the inside of the circle. Any radius drawn to a tangent is perpendicular to that tangent.
