1Reading a protractor always follows the same steps
First, line up the angle's vertex with the protractor's center point. Then line up one side with the 0 mark, and read off the mark the other side points to — that's the angle.
Line up the center → line up one side with 0 → read the other side's mark
Remember just these three steps, and you'll never get lost with any protractor.
Reading from the side lined up with 0 to the other side gives the angle
2Angles have names, too
An angle's name changes with its size. They're only split by ranges, so they're not hard to remember.
Acutegreater than 0°, less than 90°
Right angleexactly 90°
Obtusegreater than 90°, less than 180°
Straight angleexactly 180° (a straight line)
3Right and straight angles are exactly one value
Acute and obtuse are defined by a range, but a right angle (90°) and a straight angle (180°) are only called that at their exact value. 89° is still acute, and 91° is already obtuse.
A straight angle is really just a straight line.
At 180°, the two sides point in exactly opposite directions, so it looks like one straight line. It's called an angle, but it might not look like one at a glance.
At 180°, the two sides point in exactly opposite directions, so it looks like one straight line. It's called an angle, but it might not look like one at a glance.
4Try playing like this
- Open the angle slowly with the slider. Watch the exact moment it crosses from acute to right, and from right to obtuse.
- Guess the angle from the marks alone. Practice reading exact numbers in the protractor-marks quiz.
- Find angles around you. Look for acute, right, and obtuse angles in clock hands, book corners, or an open door, and sort them.