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If a transversal line meets two lines, eight angles are formed at the points of intersection as shown in the below figure. The pairs of angles $$∠1$$, $$∠2$$; $$∠3$$, $$∠4$$; $$∠5$$, $$∠6$$ and $$∠7$$, $$∠8$$ are linear pairs.

Besides, the pairs $$∠1$$, $$∠3$$; $$∠2$$, $$∠4$$; $$∠5$$, $$∠7$$ and $$∠6$$, $$∠8$$ are vertically opposite angles.

We can further classify the angles into different categories as follows:

Corresponding angles:

Observe that the pair of angles $$∠1$$ and $$∠5$$ that are marked at the right side of the transversal line $$l$$. In that $$∠1$$ lies above the line $$m$$ and $$∠5$$ lies above the line $$n$$.

Also, observe the pair of angles $$∠2$$ and $$∠6$$ that are marked on the left of the transversal line $$l$$. In that $$∠2$$ lies above $$m$$ and $$∠6$$ lies above $$n$$.

In the same way observe the pair of angles $$∠3$$ and $$∠7$$ that are marked on left of transversal line $$l$$. In that $$∠3$$ lies below $$m$$ and $$∠7$$ lies below $$n$$.

Observe the pair of angles $$∠4$$ and $$∠8$$ that are marked on the right of transversal line $$l$$. In that $$∠4$$ lies below $$m$$ and $$∠8$$ lies below $$n$$.

So all these pairs of angles have different vertices, lie on the same side (left or right) of the transversal line ($$l$$) lie above or below the lines $$m$$ and $$n$$. Such pairs are called corresponding angles.

Alternate Interior angles: Each of pair of angles named $$∠3$$ and $$∠5$$, $$∠4$$ and $$∠6$$ are marked on the opposite side of the transversal line $$l$$ and are lying between lines $$m$$ and $$n$$ are called alternate interior angles.

Alternate Exterior angles: Each pair of angles named $$∠1$$ and $$∠7$$, $$∠2$$ and $$∠8$$ are marked on the opposite side of the transversal line $$l$$ and are lying outside of the lines $$m$$ and $$n$$ are called alternate exterior angles.
Some more pairs of angles:
• Each pair of angles named $$∠3$$ and $$∠6$$, $$∠4$$ and $$∠5$$ are marked on the same side of transversal line $$l$$ and are lying between the lines $$m$$ and $$n$$. These angles are lying on the interior of the lines $$m$$ and $$n$$ as well as the same side of the transversal line $$l$$ called as co-interior angles. • Each pair of angles named $$∠1$$ and $$∠8$$, $$∠2$$ and $$∠7$$ are marked on the same side of transversal line $$l$$ and are lying outside of the lines $$m$$ and $$n$$. These angles are lying on the exterior of the lines $$m$$ and $$n$$ as well as the same side of the transversal line $$l$$ called as co-exterior angles. 