Parallel Lines

Parallel lines run in the same direction and never meet. A figure says two lines are parallel with matching arrowheads, called chevrons, drawn on both. In writing the sign is ∥: AB ∥ CD reads "AB is parallel to CD".

A line that crosses both parallels is a transversal. It makes two crossings, one on each parallel, with four angles at each. Those eight angles are what this chapter is about: because the parallels point the same way, the transversal crosses both of them at the same slant, so the two crossings are copies of each other.

As always, the marks decide. Two lines that look parallel but carry no chevrons and no ∥ statement are not known to be parallel, and none of the rules in this chapter apply to them.