In this case, the angles opposite one another are called vertical angles. As you can see, vertical angles are by definition always equal to each other. Therefore, in this case, 1 = 2, and 3 = 4. Another way to say this is that both "little angles" will equal each other and both "big angles" will equal each other. Also, note that a "little angle" plus a "big angle" will always equal 180°.

Parallel Lines

Two lines that never intersect and never get closer or farther away from one another are called parallel lines. || is the symbol for parallel lines. In the figure to the right, A || B, or line A is parallel to line B.