If we consider the situation where we have a pair of lines, lying parallel with one another and intersected by a single line at no particular angle, such as in the example below, there are a few things to remember:


The angles indicated 'a' form a pair of "alternate angles" in a sort of "Z shape" as shown by the red dotted line. Alternate angles are equal.You should be able to see that the opposite angles (the other side of the intersection) would also be represented by 'a' and will also be equal.


Parallel lines are usually shown using overlaid chevrons, where there are two or more pairs of parallel lines, this can be doubled or even trebled up so that each parallel line is shown with its counterpart >> or >>>.