Parallel Line at a Given Distance from another Line
Figure 4-2 shows another method of drawing one line parallel to another, this one being used when the second line is to be drawn at a given distance from the first. To draw a line parallel to AB at a distance from AB equal to CD, set a compass to the length of CD, and, from any points E and F on AB, strike two arcs. A line A’B’ drawn tangent to (barely touching) the arcs is parallel to AB, and located CD distance from AB.
(Source: NAVEDTRA 14069, U.S. Navy)