Introduction to Angle Measurement and the Protractor
What is an Angle?
An angle is formed when two rays (or line segments) share a common endpoint called the vertex. Angles are all around us—in the corners of your notebook, the hands of a clock, or the branches of a tree.
We measure angles in units called degrees, represented by the symbol °. A full circle contains 360 degrees.
Key angle measurements to remember:
- A right angle = 90°
- A straight angle = 180°
- A full circle = 360°
Types of Angles
- Acute angle: Less than 90° (smaller than a right angle)
- Right angle: Exactly 90° (forms an L-shape)
- Obtuse angle: Greater than 90° but less than 180° (larger than a right angle)
- Straight angle: Exactly 180° (forms a straight line)
- Reflex angle: Greater than 180° but less than 360°
- Full angle: Exactly 360° (forms a complete circle)
Using a Protractor
A protractor is a semicircular or circular measuring tool used to measure angles in degrees. Most protractors are marked from 0° to 180°, with markings in both directions.
Parts of a Protractor:
- Base line: The straight edge of the protractor
- Center point: The small hole or mark at the middle of the base line
- Scale: The numbers around the curved edge (usually two scales, inner and outer)
How to Measure an Angle:
- Place the center point of the protractor on the vertex of the angle.
- Align the base line with one ray of the angle.
- Find where the other ray intersects the protractor's scale.
- Read the measurement in degrees from the appropriate scale (make sure you're using the scale that starts from 0 where your base ray is).