A chord of a circle is a geometric line segment whose endpoints both lie on the circle. A secant line, or just secant, is the line extension of a chord. More generally, a chord is a line segment joining two points on any curve, for instance an ellipse. A chord that passes through a circle's center point is the circle's diameter.
The red segment BX is a chord
(as is the diameter segment AB).
Chords of a circle
Among properties of chords of a circle are the following:

Chords are equidistant from the center only if their lengths are equal.

A chord that passes through the center of a circle is called a diameter, and is the longest chord.

If the line extensions (secant lines) of chords AB and CD intersect at a point P, then their lengths satisfy AP·PB = CP·PD (power of a point theorem).
The area that a circular chord "cuts off" is called a circular segment.
Chord is from the Latin chorda meaning bowstring.
Chords of an ellipse
The midpoints of a set of parallel chords of an ellipse are collinear.^{[1]}^{:p.147}
Chords in trigonometry
Chords were used extensively in the early development of trigonometry. The first known trigonometric table, compiled by Hipparchus, tabulated the value of the chord function for every 7.5 degrees. In the second century AD, Ptolemy of Alexandria compiled a more extensive table of chords in his book on astronomy, giving the value of the chord for angles ranging from 1/2 degree to 180 degrees by increments of half a degree. The circle was of diameter 120, and the chord lengths are accurate to two base60 digits after the integer part.
The chord function is defined geometrically as in the picture to the left. The chord of an angle is the length of the chord between two points on a unit circle separated by that angle. The chord function can be related to the modern sine function, by taking one of the points to be (1,0), and the other point to be (cos \theta, sin \theta), and then using the Pythagorean theorem to calculate the chord length:

\mathrm{crd}\ \theta = \sqrt{(1\cos \theta)^2+\sin^2 \theta} = \sqrt{22\cos \theta} =2 \sin \left(\frac{\theta }{2}\right).
The last step uses the halfangle formula. Much as modern trigonometry is built on the sine function, ancient trigonometry was built on the chord function. Hipparchus is purported to have written a twelve volume work on chords, all now lost, so presumably a great deal was known about them. The chord function satisfies many identities analogous to wellknown modern ones:
Name

Sinebased

Chordbased

Pythagorean

\sin^2 \theta + \cos^2 \theta = 1 \,

\mathrm{crd}^2 \theta + \mathrm{crd}^2 (180^\circ  \theta) = 4 \,

Halfangle

\sin\frac{\theta}{2} = \pm\sqrt{\frac{1\cos \theta}{2}} \,

\mathrm{crd}\ \frac{\theta}{2} = \pm \sqrt{2\mathrm{crd}(180^\circ  \theta)} \,

Apothem (a)

c=2 \sqrt{r^2 a^2}

c=\sqrt{D ^24 a^2}

Angle (θ)

c=2 r \sin \left(\frac{\theta }{2}\right)

c=D \sin \left(\frac{\theta }{2}\right)

See also
References

^ Chakerian, G. D. "A Distorted View of Geometry." Ch. 7 in Mathematical Plums (R. Honsberger, editor). Washington, DC: Mathematical Association of America, 1979.
External links

History of Trigonometry Outline

Trigonometric functions, focusing on history

Chord (of a circle) With interactive animation
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