Skill: Identify perpendicular lines

Perpendicular lines are lines that meet to make a right angle. A right angle measures $90^\circ$.

To identify perpendicular lines:

1. Find where the two lines intersect.
2. Look at the angles formed at the intersection.
3. Check whether the angles are right angles, often shown by a small square.
4. If the lines are full lines, all four angles at the intersection will measure $90^\circ$.

Example

Suppose lines $\overleftrightarrow{AB}$ and $\overleftrightarrow{CD}$ intersect. The diagram shows a small square at the intersection, so one angle measures $90^\circ$.

Because the two are full lines, the other three angles at the intersection are also right angles:

$$90^\circ,\ 90^\circ,\ 90^\circ,\ 90^\circ$$

Therefore, $\overleftrightarrow{AB}$ and $\overleftrightarrow{CD}$ are perpendicular lines.

Remember: Lines do not have to be horizontal and vertical to be perpendicular. Slanted lines can also be perpendicular if they meet to form $90^\circ$ angles. Simply crossing is not enough.