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Two arms and one vertex
To show an angle, we draw two rays with the same endpoint. They are its arms, and their shared endpoint is the vertex. The angle describes the opening between the arms.
Acute, right, and obtuse angles
An acute angle is greater than 0° and less than 90°. A right angle is exactly 90°. An obtuse angle is greater than 90° and less than 180°. An angle of 180° is a straight angle.
The length of the drawn arms
Longer drawn arms do not mean a bigger angle. The opening between them matters. If you extend the drawn arms without changing their directions, the angle stays the same.
Try it yourself
Move the slider below the picture to 45°, then 90°, then 120°. Name each angle. Draw one example of each on paper.
See an example answer
45° is acute, 90° is right, and 120° is obtuse. The shared endpoint is the vertex in every example.
Check your knowledge
Choose one answer for each question. You can revise your answers and check again.
Answer key
- Vertex. Both arms start at the vertex.
- Acute. 45° lies between 0° and 90°.
- 90°. Exactly 90° makes a right angle.
- Obtuse. 120° is greater than 90° and less than 180°.
- Stays the same. The opening between the arms has not changed.