An angle is formed when lines or line segments meet.

This image shows when two lines meet or cross each other an angle is formed at the intersection, the angle is shown by a blue curve in that image.

As shown in the images you can start counting angles from 0° to 360° with respect to any line or line configurtion.

In the images a coordinate system is formed with the orgin at the junction of horizontal and vertical line intersection.

A line which cuts two different but parllel lines is called Transversal Line

As you can see in the image an inclined line is interesecting with two parllel lines thus making itself a transversal line.

If two parallel lines are cut by a transversal, each pair of corresponding angles are equal in measure.

The angles depicted by red,blue,green,yellow curves between the transversal & parllel line1 and transversal & parllel line2 are shown in the image.

If two parallel lines are cut by a transversal, each pair of alternate interior angles are equal.

If two parallel lines are cut by a transversal, then each pair of interior angles on the same side of the transversal are supplementary (i.e.. Sum of two angle is 180°)

As shown in the image the pair of interior angles shown on same sides are equal.