Types of Angles

Geometry · Axiom Academy

Learn to identify and classify angles by their measure: acute, right, obtuse, straight, and reflex. An acute angle measures less than 90°. Watch the moving ray open from the baseline and stop short of a square corner — small and sharp. Think "small and cute"! A right angle measures exactly 90°. The two rays meet in a perfect square corner — the little box in the corner is the universal mark for "this is a right angle." It forms an "L" shape. An obtuse angle measures more than 90° but less than 180°. The ray opens past the square corner but never straightens out into a line. "Ob-tuse" = larger! Wider than a right angle, not yet straight A 135° angle is obtuse: it has opened well past the 90° corner (you can picture the right-angle box tucked inside it) but the two rays still form a bend, not a straight line. A straight angle measures exactly 180°. The moving ray opens until it points in the exact opposite direction — the two rays line up into a single straight line. Exactly a half-turn — a straight line A reflex angle measures more than 180° but less than 360°. Instead of taking the short way around, the angle is measured the long way — it goes "around" the back. Watch the arc sweep past the straight line and keep going. More than straight, less than a full turn You can now classify any angle from its measure alone — read the number, find its range, name the type. Scroll up to revisit any step.

This is the written version of the interactive lesson above. See the full Geometry course.