A chord is a line segment connecting two points on a circle.
A chord is a fundamental element of circle geometry. The longest chord of a circle is the diameter, which passes through the center. Chords that are equidistant from the center have equal lengths. The perpendicular bisector of a chord always passes through the center of the circle. Chords are used in music (string instruments), engineering (arches), and geometry theorems.
The study of chords was essential in early Greek trigonometry. Hipparchus of Nicaea (190-120 BCE) created the first known chord table, laying the foundation for trigonometry. Ptolemy's "Almagest" (150 CE) contained a chord table for angles in a circle. The chord function is historically the oldest trigonometric function, predating sine.