Parallel lines are lines in the same plane that go in the same direction and never intersect. When a third line, called a transversal, crosses these parallel lines, it creates angles. Some angles are equal, like vertical angles (opposite angles) and corresponding angles (same position at each intersection). Created by Sal Khan.
Two parallel lines are intersected by a transversal line, producing 8 angles. Observe the angle pairs: Corresponding, Vertical, Alternate Interior, Alternate Exterior, Same Side Interior, and Same Side Exterior.
In geometry we are concerned with the nature of these shapes, how we define them, and what they teach us about the world at large--from math to architecture to biology to astronomy (and everything...