If we add a third line that intersects the two parallel lines (those lines that could never cross), we will obtain various types of angles. To classify these angles we must observe if they are: above the line - the pink part below the line - the light blue part to the right of the line - the red part to the left of the line - the green part

The sum of consecutive angles located between parallel lines is equal to $180$. They are called consecutive angles because:

they are on the same side of the transversal

but they are not on the same "level" in relation to the line

Here are some examples of consecutive angles:

The two marked angles are on the same side of the line, but at a different height, therefore, they are consecutive angles. Observe: The angles painted in red in the illustration above are external consecutive angles since they are on the outer side of the parallel lines The internal consecutive angles are on the inner side of the parallel lines:

Angles on Parallel Lines

Now to practice!

Give an example according to the illustration of:

Alternate angles

Corresponding angles

Vertically opposite angles

Adjacent angles

Consecutive interior angles

Consecutive exterior angles

Solution:

Examples of alternate angles $1, 8$ Both are on different sides and levels, therefore, they are alternate.

Examples of corresponding angles $8,4$ Both are on the same side and at the same level or floor, therefore, they are corresponding.

Examples of vertically opposite angles $1,4$ Both share the same vertex and are located opposite each other, therefore, they are vertically opposite angles.

Examples of adjacent angles $7,8$ Both are on the same line and are located next to each other, therefore, they are adjacent.

Examples of exterior alternate angles $1,7$ Both are on the same side, but not at the same level. In addition, they are located on the outside of the line, therefore, they are exterior alternate angles.

Examples of interior alternate angles $3,5$ Both are on the same side, but not at the same level. In addition, they are located on the inside of the line, therefore, they are interior alternate angles.

Another exercise:

What are the marked angles called in the illustration?

Solution

The marked angles are alternate They are located on different sides and heights, therefore, they are alternate.

Another exercise:

What are the angles shown in the illustration called?

Solution

The indicated angles are consecutive They are on the same side of the line, but at different heights, therefore, they are external consecutive angles.

Another exercise:

What are the angles shown in the illustration called?

Solution

The indicated angles are adjacent They are on the same blue line and are next to each other, therefore, they are adjacent angles.