Animated lessons
Angles, from zero to reflex.
Four short lessons, each one picture-first. Watch the rays move, then try it yourself in the protractor or the quiz.
Lesson 1
What is an angle?
An angle measures how far one ray has turned away from another, around the point they share — the vertex. We measure the turn in degrees: a full circle is 360°, a quarter turn is 90°, and a half turn is 180°.
Watch the blue ray sweep around the vertex: every position it passes is a different angle from the black ray.
Lesson 2
The six angle types
Angles get their names from how far the ray has turned. Learn the cut-offs — 90° and 180° — and every classification becomes easy.
Zero
exactly 0°Acute
1° – 89°Right
exactly 90°Obtuse
91° – 179°Straight
exactly 180°Reflex
181° – 359°Lesson 3
Complementary & supplementary pairs
Some angles come in pairs that always add up to the same total. Spot the shape and you know the sum instantly.
Complementary
adds to 90°Two angles that fill a corner together are complementary: 35° + 55° = 90°.
Supplementary
adds to 180°Two angles that fill a straight line together are supplementary: 120° + 60° = 180°.
- 1
Find the vertex
The vertex is the corner where the two rays meet. Every measurement starts there.
- 2
Line up the crosshair
Drag the protractor so its center crosshair sits exactly on the vertex.
- 3
Align the baseline
Rotate the protractor until its straight baseline lies along one of the rays.
- 4
Read the scale
Follow the second ray out to the scale. If it points left, use the inner numbers; if it points right, use the outer numbers.
Inner or outer scale?
Both scales are printed on every protractor — that’s the number-one source of mistakes. The rule: start counting from the ray you aligned with the baseline, at 0°. If your baseline ray points right, start from the right-hand 0 (outer scale). If it points left, start from the left-hand 0 (inner scale).
Practice makes it stick
Try what you learned.
Ten questions, instant feedback, and XP for every correct angle.