What are Perpendicular Lines?

Lines AC and DB intersect, or cross, at point E
The little box means the angle it marks is a right angle, so it measures 90˚
Angles AED and BEC sit across from each other, so they are vertical angles
Vertical angles are always congruent
If we rotate perpendicular lines a quarter of a turn, they still look exactly the same

1. Any angle that is 90˚ is called a 'right angle'
1. Right angles are always 90˚
1. This is true for ANY set of intersecting lines, not just perpendicular lines
1. We can subtract these two angle measurements from 360° to find out the measures of the other two angles
1. Angles AEB and DEC also sit across from each other, so they are also vertical angles
2. Vertical angles are congruent
3. Since 180˚ is all that's left to split between the two remaining congruent angles, we can just divide 180 by 2 to find their measure
4. So each angle measures 90°
1. A 'quarter turn' just means we rotate the diagram 90˚
2. Since our figure is made up of only 90˚ angles, it'll always look the same if we rotate it 90˚
1. The angles at an intersection must add up to 360˚
2. Since there are four congruent angles adding up to 360˚, we can just divide 360 by 4 to find the measurements of each angle
3. Again, each angle measures 90°

What are perpendicular lines?