Introduction to angles created and their measures when two parallel lines are intersected by a transversal. Teacher can use hand out as notes to begin building understanding of different types of angle and specific vocabulary. Best results when these notes are supplemented by anchor charts with vocabulary and images of the angels are posted in the classroom.
Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.