2. Angles formed by transversal lines

If a transversal line meets two lines (parallel or unparalleled 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: