1. When lines intersect to form a right angle, they are called perpendicular lines.

2. Planes, segments and rays can also be perpendicular to one another if they intersect at 90 degree angles.

3. Coplanar lines that do not intersect are called parallel lines.
Planes that do not intersect are parallel planes
If lines are not in the same plane and do not intersect, they are skew lines.

4. Theorem 5-1
If 2 parallel lines are cut by a third plane, then the lines of intersection are parallel

5. Theorem 5-2
If 2 lines in a plane are perpendicular to the same line, then they are parallel to each other

6. Theorem 5-3
In a plane, if a line is perpendicular to one of 2 parallel lines, then it is perpendicular to the other one.

7. Theorem 5-4
If 2 lines are perpendicular, then they form congruent adjacent angles.

8. Theorem 5-5
If 2 lines form congruent adjacent angles, then they are perpendicular

9. Theorem 5-6
ALL RIGHT ANGLES ARE CONGRUENT

10. Postulate 10- The Parallel Postulate
Through a point not on a line, there exists exactly one line through the point that is parallel to the line.

11. HINT****************
The distance between 2 parallel lines is the same at every point.

12. Theorem 5-7
If 2 lines are parallel to the same line, then they are parallel to one another.