1. If two lines intersect, then the vertical angles are always **congruent**. This is called the **Vertical Angles Theorem**. 2. If parallel lines are cut by a transversal, then same-side interior angles are **supplementary** (their measures add up to 180 degrees). This is called the **Same-Side Interior Angles Theorem**. 3. If parallel lines are cut by a transversal, then alternate interior angles are **congruent**. This is called the **Alternate Interior Angles Theorem**. 4. The definition of parallel lines is lines that are **coplanar and do not intersect, no matter how far they are extended**.