A line that links two points on a circle or curve. (pronounced "cord")

Try this Drag either orange dot. The blue line will always remain a chord to the circle.

The blue line in the figure above is called a "chord of the circle c". A chord is a lot like a secant, but where the secant is a line stretching to infinity in both directions, a chord is a line segment that only covers the part inside the circle. A chord that passes through the center of the circle is also a diameter of the circle.

The perpendicular bisector of a chord always passes through the center of the circle. In the figure at the top of the page, click "Show Right Bisector". Then move one of the points P,Q around and see that this is always so. This can be used to find the center of a circle: draw one chord and its right bisector. The center must be somewhere along this line. Repeat this and the two bisectors will meet at the center of the circle. See Finding the Center of a Circle in the Constructions chapter for step-by-step instructions.

If two chords of a circle intersect, the intersection creates four line segments that have an interesting relationship. See Intersecting Chord Theorem.

- Circle definition
- Radius of a circle
- Diameter of a circle
- Circumference of a circle
- Parts of a circle (diagram)
- Semicircle definition
- Tangent
- Secant
- Chord
- Intersecting chords theorem
- Intersecting secant lengths theorem
- Intersecting secant angles theorem
- Area of a circle
- Concentric circles
- Annulus
- Area of an annulus
- Sector of a circle
- Area of a circle sector
- Segment of a circle
- Area of a circle segment (given central angle)
- Area of a circle segment (given segment height)

- Basic Equation of a Circle (Center at origin)
- General Equation of a Circle (Center anywhere)
- Parametric Equation of a Circle

- Arc
- Arc length
- Arc angle measure
- Adjacent arcs
- Major/minor arcs
- Intercepted Arc
- Sector of a circle
- Radius of an arc or segment, given height/width
- Sagitta - height of an arc or segment

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