Consider two parallel lines that are intersected by a transversal, and when it crosses the two others, many different angle pairs are formed. See the special properties of the angle pairs in this interactive.

[Free Registration/Login Required] Explore the relationships between parallel lines and the rules of angle measurements.

This site defines parallel lines, parallel lines cut by a transverse, and congruent triangles. It also contains links for related terms.

[Free Registration/Login Required] This flipchart is a look at angles in geometry. Drawing, labeling, measuring, special angles: (right, straight, acute, obtuse) and congruence.