Label the Parts of the Angle – Vertex, Sides, Arc and Kinds

All 12 parts on the diagram

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One angle 6

Vertex The point where the two sides meet. Here it is B.
Side One of the two rays that form the angle. Each starts at the vertex.
Arc A curve drawn between the sides to mark the angle.
Angle measure The size of the angle, in degrees. This one measures 50°.
Interior The region between the two sides of the angle.
Exterior The region outside the two sides of the angle.

Angles by size 6

Acute angle Less than a right angle: under 90°.
Right angle Exactly 90°. A small square marks it.
Obtuse angle Wider than a right angle, under a straight angle: 90° to 180°.
Straight angle Its sides point opposite ways and make one straight line: 180°.
Reflex angle More than a straight angle, under a full turn: 180° to 360°.
Full angle One complete turn, back to the start: 360°.

How this list was compiled: Every name was read on 2026-10-01 in at least two of these. (1) Wikipedia, English edition, Angle, https://en.wikipedia.org/wiki/Angle (the sides and the vertex; the interior and the exterior of the angle; an arc of a circle centered on the vertex, between the sides; naming by one or three points; acute, right, obtuse, straight and reflex angles with their degrees; full angle, also complete angle, round angle or perigon). (2) California Department of Education, California Common Core State Standards: Mathematics, https://www.cde.ca.gov/be/st/ss/documents/ccssmathstandardaug2013.pdf, read with pdftotext (grade 4: angles form wherever two rays share an endpoint; an angle is measured by the circular arc between the rays; a one-degree angle turns through 1/360 of a circle; an angle measure of n degrees; right, acute and obtuse angles). (3) Plane Geometry, a textbook of 1913 in the public domain, https://archive.org/details/planegeometry00wentrich (the sides and the vertex; an angle read with the vertex letter between the others; a curve drawn to show the angle meant; size depends on the amount of turning, not on the length of the sides; straight, right, acute, obtuse and reflex angles; the perigon; the degree; 90, 180 and 360 degrees). (4) For the interior and the exterior of the angle, three school books read by full-text search: a class 6 mathematics chapter, https://archive.org/details/femh107, College Geometry, https://archive.org/details/collegegeometry00denn, and Basic Geometry (1999), https://archive.org/details/basicgeometry0000unse_j9z0. (5) For the name full angle, Maths Made Easy (1995), https://archive.org/details/mathsmadeeasy0000boot (a full angle is subdivided into 360 degrees), and All Math Words Dictionary (2012), https://archive.org/details/allmathwordsdict0000davi, both read by full-text search. Hints, detail lines and the order are our own. Licence: own work

The drawing: Drawn for this site by script, 2026-10-01. Own work. Licence: own work

How it works

Label the parts of the angle ABC: its vertex, its two sides, the arc that marks it, its measure in degrees, and its interior and exterior. A second drawing sorts six angles by size, from acute to full.

Read the full guide

One angle

Two rays that share an endpoint form an angle. The shared point is the vertex and the rays are the sides. This one is angle ABC, read with the vertex letter B in the middle. The arc shows which angle is meant, and 50° is its measure: one degree is 1/360 of a full turn.

Angles by size

An acute angle is less than 90°, a right angle is exactly 90° and an obtuse angle lies between 90° and 180°. A straight angle is 180°, a reflex angle is more than 180°, and a full angle is 360°.

Longer sides do not make a bigger angle. The size depends only on the amount of turning from one side to the other.

Questions and answers

What are the parts of an angle?

A vertex, the point where its two sides meet, and the two sides, which are rays. The region between the sides is the interior and the rest is the exterior. An arc near the vertex marks the angle.

What are the sides of an angle called?

They are called the sides of the angle. Each side is a ray that starts at the vertex. In angle ABC the sides are the rays BA and BC.

How do you name an angle?

With three letters, the vertex letter in the middle: angle ABC or angle CBA. An angle can also be named by its vertex letter alone, as angle B.

What is a reflex angle?

An angle greater than a straight angle and less than a full turn, so between 180° and 360°. Two rays from one point make two angles; unless told otherwise, the smaller one is meant.