How to name parts of a circle from a diagram

Key concept: Circle vocabulary gets easier when you sort each segment by what it touches. A radius reaches the center, a chord connects two points on the circle, and a tangent touches once.

Step by step

Here are the labels based on the diagram descriptions:

• Circle: $C$
• Two radii: $AC$ and $CD$
• Two chords: $AD$ and $BD$
• Diameter: $AB$
• Secant: $GE$

For the last part, a tangent is a line that touches the circle at exactly one point, and that point is called the point of tangency.

So if the diagram shows a tangent line, name the line and the single point where it touches the circle.

Pitfall alert

Students often mix up a chord and a secant. A chord stays inside the circle and connects two points on the circle; a secant cuts through the circle and extends outside it. Another common slip is calling any line that touches the circle a tangent without naming the point where it touches.

Try different conditions

If the line touched the circle at two points, it would be a secant instead of a tangent. If a segment goes from the center to any point on the circle, it is a radius no matter which direction it points.

Further reading

radius, chord, tangent