Types of Angles: Acute, Right, Obtuse, Straight, Reflex, and Full Turn

Grade 4 first: types of angles

Yes, the types of angles lesson belongs in Grade 4 geometry. Students usually learn angle names before they study one special angle in more detail.

This lesson should come first because it gives the big picture. After this, a focused page about right angles is easier to understand.

What does angle type mean?

An angle is made when two rays meet at one endpoint. The endpoint is called the vertex.

An angle type is the name we give an angle after we look at its size in degrees.

Small openings, square corners, straight lines, and large turns all get different names.

Types of angles chart

Use this chart as a quick guide. Some angle types are exact sizes, and some angle types are ranges between two sizes.

The degree sign, °, means degrees. Degrees tell how wide the angle opens or how far it turns.

Other names for angle types

Some angle types have more than one name. The meaning stays the same, even if the wording changes.

Zero angle: also called a 0-degree angle. Some books may say null angle, which means no opening.

Acute angle: sometimes described as a small angle or sharp angle, because it is less than 90°.

Right angle: also called a 90-degree angle. It is the square-corner angle used in the focused right angle lesson.

Obtuse angle: sometimes described as a wide angle because it is more than 90° but less than 180°. For more detail, review obtuse angle.

Straight angle: also called a 180-degree angle or a flat angle. It looks like one straight line.

Reflex angle: sometimes called a re-entrant angle in higher geometry. It is more than 180° but less than 360°.

Full turn: also called a 360-degree angle, complete angle, or full angle.

Visual examples

The drawings below show how the opening changes from small to large.

Notice that right, straight, and full turn angles have exact sizes. Acute, obtuse, and reflex angles cover many possible sizes.

How to name an angle

Step 1: Find the angle size in degrees.

Step 2: Compare the number with 0°, 90°, 180°, and 360°.

Step 3: Choose the angle name that matches the size rule.

The boundary numbers matter. For example, 90° is right, but 91° is obtuse. If a problem says 180-degree angle or flat angle, it means straight angle.

Worked example

Problem: An angle measures 126°. What type of angle is it?

Step 1: 126° is more than 90°.

Step 2: 126° is less than 180°.

Answer: The angle is an obtuse angle.

Example sorting chart

Here are sample measurements and their angle names.

15° is acute. 90° is right. 142° is obtuse. 180° is straight. 275° is reflex. 360° is a full turn.

When you sort angles, read the number carefully before you guess from the picture.

Real-life angle clues

A door opened a little can show an acute angle. A book corner often shows a right angle. A laptop opened wide can show an obtuse angle.

A flat road or ruler edge can help you imagine a straight angle. A large turn around a point can show a reflex angle or a full turn.

Exact angles and range angles

Some names are exact. Zero, right, straight, and full turn angles each have one exact degree measure.

Other names are ranges. Acute, obtuse, and reflex angles can have many different degree measures.

This is why 40° and 70° are both acute, even though they are not the same size. Two angles can also work as a pair, such as complementary angles that total 90°.

Common mistakes

Do not call 90° acute. Acute angles must be less than 90°.

Do not call 180° obtuse. Obtuse angles must be less than 180°.

Do not call 360° reflex. A reflex angle stops before a full turn.

Do not use the drawing alone if a degree measure is given. The number is the stronger clue.

Quick practice

1. 48° is acute because it is more than 0° and less than 90°.

2. 90° is right because it is exactly 90°.

3. 119° is obtuse because it is more than 90° and less than 180°.

4. 180° is straight because it makes a straight line.

5. 310° is reflex because it is more than 180° and less than 360°.