-- On a flat, 2-dimensional (x-y) graph, the locus of all points that are
4mm from a given line consists of two more lines, one on each side
of the given line, and all three lines are parallel.
-- If we want to talk about 3-dimensional space, then the locus of all
points that are 4mm from a given line is a cylinder, with radius of 8mm,
and the given line is the axis of the cylinder.