When parallel lines are crossed by a transverse line, pairs of angles are formed that have special names like corresponding angles, alternate interior angles, alternate exterior angles, and consecutive interior angles. If any pair of these special angles are equal or add up to 180 degrees, then the lines crossed by the transverse line are parallel.

3. Q.4) When parallel lines get crossed by another line (which is
called a ________), you can see that many angles are the
same, as in this example:
These angles can be made into pairs of angles which
have special names.

6. Parallel Lines

8. Pairs of Angles
Transverse

10. Q.6) Some of those special pairs of
angles can be used to test if lines really are
parallel:
If Any Pair Of ... Example
Corresponding angles
are equal
a = e,
______
Alternate Interior
Angles are equal
c = f,
_______
Alternate Exterior
Angles are equal
b = g,
_______
Consecutive Interior
Angles add up to
180°
d + f =
180°,
__________
... then the lines are Parallel.
Examples