Now that you know what an angle measures, you can sort angles by comparing them with a right angle, the square corner of a sheet of paper.
Key idea
An acute angle is less than $90^\circ$. A right angle is exactly $90^\circ$ (a square corner, often marked with a small square). An obtuse angle is between $90^\circ$ and $180^\circ$. A straight angle is exactly $180^\circ$: its rays form a line.
Why it works
A right angle is a quarter turn. Anything that turns less than a quarter turn fits inside a square corner (acute); anything more sticks out past it (obtuse). A half turn, $180^\circ$, points the rays in opposite directions, which makes a straight line.
Worked example: Sort by measure
Is a $135^\circ$ angle acute, right, obtuse or straight?
- $135^\circ$ is more than $90^\circ$ and less than $180^\circ$.
Answer: Obtuse
Worked example: Clock hands
At 3:00, what kind of angle do the clock hands make?
- From 12 to 3 is a quarter turn: $90^\circ$.
Answer: A right angle
Common mistake
Judging by how the angle is turned on the page. A right angle tipped on its side is still $90^\circ$, and an acute angle with long rays is still acute. Hold a square corner against it to check.