When constructing parallel lines with a compass and straightedge, how should you start the construction?

Question

anonymous · Answer

Open the compass and mark two points of intersection between arcs from the given line.

larseighner · Answer

There are several possible constructions, depending on what your are given and what you want, but the main concept is that parallel lines will have the same angle with any transverse line.  If you aren't given a transverse line, it is usual to construct a perpendicular, because it is the easiest construction.