Postulate 11 (Parallel Postulate): If two parallel lines are cut by a transversal, then the corresponding angles are equal (Figure 1).
![](http://s3.amazonaws.com/prod-hmhco-vmg-craftcms-public/_cliffsnotes/assets/18008.jpg)
Figure 1 Corresponding angles are equal when two parallel lines are cut by a transversal.
This postulate says that if l // m, then