Angles are grouped by their size. Knowing the names makes it faster to describe and reason about shapes.
An acute angle is less than 90° — it looks sharp and pointy. A right angle is exactly 90° — the classic square corner. An obtuse angle is between 90° and 180° — wider and more open. A straight angle is exactly 180° — it looks like a flat line. A reflex angle is between 180° and 360° — it's the "long way around".
Remember
Right angles are marked with a small square in the corner. If you see that symbol, the angle is exactly 90° — no measuring needed.
Real World
A slice of pizza is roughly an acute angle. The corner of a room is a right angle. An open door swung almost all the way back creates an obtuse angle.
Feeling ready?
Put this concept to work with a practice workout.