Parallel Lines Concept Two lines AB and CD are parallel lines, if they are on the same plane have no points in common, meaning that regardless of how far they extend, they never intersect. Properties of parallel lines a. Reflexive: Every line is parallel to itself b. Symmetric: If AB || CD c. Transitive: If AB || CD and CD || EF AB || EF. CD||AB Symetric property Transitive property Transversal Angles formed by a transversal a. Supplementary angles b. Vertical angles c. Interior and exterior angles d. Corresponding angles e. Alternate interior angles f. Alternate exterior angles