What is an angle?
Push a door open, slowly. Watch the gap grow.
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Step 1 of 4
- Closed. The two arms lie on top of each other.
- A small turn opens up.
- A quarter turn — the moving arm points straight up.
- A half turn — one straight line through the vertex.
The rule
An angle is the amount of turn between two rays. The rays share one start point, the vertex. The rays are the arms.
Name an angle with three letters. The vertex letter goes in the middle. The sign ∠ means angle. An angle with arms OA and OB is ∠AOB. Its vertex is O.
Example
A door hinge is the joint the door turns on. The hinge is the vertex. The wall and the door are the arms.