When two lines are crossed by another line (which is called the Transversal), the pairs of angles:
• on one side of the transversal
• but inside the two lines
are called Consecutive Interior Angles.
d and f are Consecutive Interior Angles.
c and e are Consecutive Interior Angles.
When the two lines are parallel, any pair of Consecutive Interior Angles add to 180 degrees.
Play with it below (try dragging the points):
../geometry/images/geom-line-para.js?mode=conint