Basic Angle Relationships
When lines intersect, they create specific angle relationships worth knowing. Vertical angles are angles that appear across from each other when two lines cross - these angles are always congruent (equal in measure). Adjacent angles share a side and are on the same line.
Two important angle pairs to remember are complementary and supplementary angles. Complementary angles are any two angles whose measures add up to 90° (like 41° and 49°). Supplementary angles are any two angles whose measures add up to 180° (like 91° and 89°).
When parallel lines are cut by another line (called a transversal), they create eight angles. These angles form special relationships that you'll explore on the next page.
Quick Tip: To remember complementary vs. supplementary angles, think "C" for Complementary = Corner (90°) and "S" for Supplementary = Straight line (180°).
