Angles and Parallel Lines
Because of the symmetry of parallel lines, there are three important angle properties involving parallel lines and a transversal.
Types of angles 
Diagram

Property

Abbreviation

Corresponding angles 
Corresponding angles are equal. e.g. a = b 
(corr. ∠'s on lines) 

Alternate angles 
Alternate angles are equal. e.g. c = d 
(alt. ∠'s on lines) 

Cointerior angles 
Cointerior angles are supplementary. e.g. d + e = 180° 
(coint. ∠'s on lines) 
Each diagram has several pairs of corresponding, alternate and cointerior angles.