Parts of a Circle Diagram – Radius, Chord, Arc, Sector

All 10 parts on the diagram

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Start here: 8 main parts 8

Center The point the same distance from every point on the circle.
Radius A line segment from the center to any point on the circle. Also its length, which is half a diameter.
Diameter A chord through the center, the circle's longest chord. Twice the radius.
Chord A line segment whose ends both lie on the circle. It divides the circle into two segments.
Circumference The distance once around the circle: its perimeter.
Arc Any connected part of the circle. Two points on a circle make two arcs that together make the whole circle.
Sector The region between two radii and the arc joining their ends. A slice of pie.
Tangent A straight line that touches the circle at exactly one point.

Points and lines: 6 1

Secant A straight line that crosses the circle at two points. A chord extended both ways.

The edge and its regions: 4 1

Segment The region between a chord and an arc joining its ends.

How this list was compiled: Read 2026-09-26. Wikipedia, Circle, section Terminology, https://en.wikipedia.org/wiki/Circle : arc (any connected part of a circle; two end points give two arcs making the whole circle); centre (the point equally far from each point of the circle); chord (a segment with both ends on the circle, dividing it into two segments); circumference (the length once around the circle); diameter (a chord through the centre, the longest chord, twice the radius); radius (a segment from the centre to the circle, or its length, half a diameter); sector (the region bounded by two radii and an arc); segment (the region a chord and one of its arcs bound); secant (an extended chord meeting the circle in two points); tangent (a line with one point in common with the circle). Licence: own work

The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work

How it works

A parts of a circle diagram for geometry: one circle with its ten named parts, from the center and radius to the sector, segment, tangent and secant. Start with the 8 main parts.

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Lines through and across

A radius runs from the center to the edge; a diameter goes right across through the center and is twice as long. A chord joins any two points of the circle. A secant is a chord extended both ways, and a tangent touches the circle at one point.

The edge and the regions

The circumference is the whole edge; an arc is part of it. A sector is a pie slice between two radii, and a segment is the piece cut off by a chord.

Sector or segment? A sector reaches the center, like a slice of pie; a segment never touches the center.

Questions and answers

What is the difference between a chord and a diameter?

A diameter is a chord that passes through the center. It is the longest chord a circle has, twice the length of the radius.

What is the difference between a tangent and a secant?

A tangent touches the circle at exactly one point. A secant cuts through it, crossing at two points.

Is the circumference the same as the circle?

The circumference is the length once around the circle's edge. In everyday speech people also call the edge itself the circumference.