The Parallel Postulate

Postulate 11 (Parallel Postulate): If two parallel lines are cut by a transversal, then the corresponding angles are equal (Figure 1).



Figure 1 Corresponding angles are equal when two parallel lines are cut by a transversal.


This postulate says that if l // m, then

  • m ∠1 = m ∠5
  • m ∠2 = m ∠6
  • m ∠3 = m ∠7
  • m ∠4 = m ∠8