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3. Pair of interior angles on the same side of transversal: The pair of interior angles on the
same side of the transversal are also called pair of consecutive interior angles.
In the figure, ∠2 and ∠5, and ∠3 and ∠8 are the pairs of interior angles on the same side
of the transversal.

Quick Check
Write the name of the following pairs of angles:
(a) (b) (c) (d) (e) (f)

1 5 9
3 4
2 7 8 10
6 1112

Parallel Lines

If two lines in a plane do not intersect each other even if produced indefinitely l
in both directions, then the lines are called parallel lines. Parallel lines are
always equidistant from each other. If l and m are two parallel lines, it is m
symbolically written as l || m and read as ‘l is parallel to m’. Following are some examples of parallel
lines in day-to-day life.

l l
m
m l

m
Angles Made by a Transversal with Two Parallel Lines

Let LM be a transversal to two parallel lines AB and CD which intersect the two lines at points Q
and P respectively. Mark all the angles as 1, 2, 3, . . . , 8.
If a transversal intersects two parallel lines, then L
• each pair of corresponding angles are equal. P
(a) ∠1 = ∠5 (b) ∠4 = ∠8 C 4 1 D
(c) ∠2 = ∠6 (d) ∠3 = ∠7 3 2
• each pair of alternate interior angles are equal. Q
(a) ∠2 = ∠8 (b) ∠3 = ∠5 8 5
• each pair of interior angles on the same side of the transversal A 7 6 B
are supplementary. M
(a) ∠2 + ∠5 = 180° (b) ∠3 + ∠8 = 180°
We can easily remember these results by the following shapes.
The F-shape stands for corresponding angles.

t
t

t

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