Transversal line

From Freepedia

In geometry, a transversal line is a line that passes through two or more other coplanar lines at different points, especially when the other coplanar lines are also parallel.

If a is parallel to b, and transversal t crosses a with angle θ, then it must also cross b with angle θ, such that the sum of all internal angles is 180 degrees, the sum of all external angles is 180 degrees, and all corresponding angles (aka alternate angles) are congruent.