The obsession of the angle between two lines

I presume that you are talking about two straight lines.

We know that two straight lines intersecting each other will form four angles.

Each angle is the same as the opposite one, so there are always two pairs of angles.

Now, there are only two interesting angles as the other two are repeated.

Cheerfully, the sum of the two interesting angles equals 180degrees.

Therefore, A (one angle) is equal to 180degrees-A.
(Notice that one angle is acute and another is obtuse)

Yet, we see that one angle is sufficiently enough to show the relationship between two lines.

To make life easier, we are likely to use the acute angle since it is less than 90degrees.



I hope I didn't answer some stupid things which you did not desire.