If you're drawing a road on a map, and that road happens to be straight, it's easy to draw its two sides parallel: you just move the ruler. But most roads twist and turn, and drawing their sides perfectly parallel cannot be done with a ruler. And yet even before computers came along, no matter how windy their roads, cartographers appear to have had no difficulty drawing both sides of their roads perfectly parallel (or I should probably say equidistant). I cannot begin to imagine a mechanical way to do this. Can anyone tell me?

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    I'm betting a sufficiently skilled draftsman could draw lines that are indistinguishable from equidistant simply by eye. Commented Oct 20, 2015 at 14:53
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    The term we use for this in cartography is "line casing" or "cased lines".
    – smithkm
    Commented Oct 22, 2015 at 2:13

6 Answers 6


Parallel Pens with special split nibs or something similar. Sorta like using two pens taped together into one pen with two tips

An example can be found on the Scribblers calligraphy website:

enter image description here

  • Attribution needed indeed. Would be nice if you also add information on how to keep it right-angled inside corners.
    – phresnel
    Commented Oct 20, 2015 at 8:13
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    @phresnel I expect there will always be some penmanship skill required for this, regardless of the tool used.
    – Gusdor
    Commented Oct 20, 2015 at 10:46
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    @Gusdor I was going to make the same comment. Just because the nib points are a fixed width apart doesn't mean that the lines end up the way you want them. If you don't turn evenly, you won't end up with equidistant edges. Commented Oct 20, 2015 at 14:14
  • I never even thought of this solution. It would require a vey steady hand, but in principle it's just about feasible, especially for a low budget solution. It would be difficult turning it around corners. But I never even thought of this solution at all. Thank you!
    – Stephen F
    Commented Oct 21, 2015 at 13:18
  • Good answer .. in addition to Ruling Pens
    – Scott
    Commented Oct 22, 2015 at 19:46

stereo plotter

Airphotos have been the source materials for a lot of maps for a long time. To result in a nice exact machine made look they would use dials to help draw the lines on transparent mylar sheets. The reason those maps had roads that were nice and parallel was the machines and skilled operators. Parallel Pens and parallel ruler are useful for other map making methods.

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    Thank you for such a gorgeous photo! Did people really invent all that just to draw maps? Now I begin to get some little insight into what goes (or used to go) into some of those aesthetically beautiful old maps, such as Philip's atlases. Thank you for finding that, it's an amazing photo.
    – Stephen F
    Commented Oct 21, 2015 at 13:15
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    "Just draw maps"? Maps are one of the primary pillars of human culture.
    – Yorik
    Commented Oct 21, 2015 at 15:42
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    "Airphotos have been the source materials for a lot of maps for a long time" For a pretty short time as a fraction of the period for which human beings have been making maps! Commented Oct 22, 2015 at 11:22
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    Not to mention, that "Mapmaking" is one of the first scientific discoveries (technical advances) found, when you play any Civilization game! :>
    – trejder
    Commented Oct 23, 2015 at 10:14
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    It was invented to make maps. A google search for Analog photogrammetry or analog stereoplotter will bring up a number of images of different machines. There is a pretty good variety of designs.
    – exploregis
    Commented Oct 25, 2015 at 2:41

Parallel ruler

There is always the parallel ruler for lines that are further apart.

A more modern version would be those with rollers but that is not as good in accuracy as a fixed mechanical design.

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    Your answer doesn't account for "But most roads twist and turn, and drawing their sides perfectly parallel cannot be done with a ruler", but a +1 for effort.
    – Pharap
    Commented Oct 20, 2015 at 8:48

Two other methods:

  1. Engineering drawings (e.g. highway plans) usually use curves with defined radii, so drawing those is mostly a matter of using a compass and creating the geometry from the same instructions for both sides. (Edit: I say "same instructions" because roads are first defined by their centerline path, then parallel stuff is based off of that. So for a highway with a 200-foot right-of-way, you'd copy the straight sections on each side, 100 feet away from the center. For curves you would add 100 feet to the radius when drawing the outside of the curve, and you'd subtract 100 feet from the centerline to do the inside of the curves.)

(Edit: as pointed out in the comments, the following method gives 2 identical curves rather than 2 parallel curves.)

  1. There's another method using the Lesbian rule (named after the island of Lesbos, where they originally got the material), aka the flexible curve. Descriptions of both are on Wikipedia (here and here respectively). This isn't as handy for small squigglies, but it's great if you can form a gentle/large curve, trace one side, move the rule, and trace the other side.
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    Hmm, I never thought of a Lesbian rule. It wouldn't be perfect but it sounds like a good solution if you're on a low budget, or if the quality doesn't have to be exact. What I mean is, in theory, on the outside of a bend the radius would have to be slightly increased, and on the inside of a bend it would have to be slightly reduced. That would be a fiddly process. And any imperfections would not be very obvious. And it would be vastly cheaper than buying a machine like the one in the eye-popping photo and hiring someone who can operate it.
    – Stephen F
    Commented Oct 21, 2015 at 13:26
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    If you want to draw to curved lines at constant distance from each other, they must have different radii. You can't "use the same instructions for both sides." Commented Oct 22, 2015 at 11:24
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    Why trace and move? That won't work (compare writing with a broad nib--wide in parts of the curve and narrow in others). Surely the idea is to leave the rule in place and trace along both sides. You just have to get one that is the right width. Commented Oct 25, 2015 at 10:26
  • I searched "Lebsian rule" on google but the results wasn't what I was expecting for -_-
    – snoob dogg
    Commented Sep 26, 2018 at 16:35

The answer that @AMontpetit provides shows the pen and ink drawing tools used to draw parallel lines. There are also scribing tools that will draw these lines. The tools have a pivot built into them, allowing you to keep an equal distance between the lines when going around curves. The tools could have a single blade or multiple blades for parallel lines. I used these tools in my first cartographic jobs a few decades ago...

See this video for an example: https://youtu.be/Ovu513papoc?t=1365

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    I would think another approach would be to use one width of brush or marker to draw a wider line, and then a narrower brush to paint the middle with a lighter opaque color. On a lot of maps, the way lines join would be consistent with producing them that way, though I have no idea if that's how they were actually done. What would your experience suggest?
    – supercat
    Commented Oct 21, 2015 at 15:01
  • That video of those ingenious scribing tools (youtu.be/Ovu513papoc?t=1365) shared by kenbuja on Oct 21 at 13:37 is gorgeous. Thank you so much. The man seems to be dragging a tiny cutter, almost like pulling a miniature trolley. I suppose this naturally keeps the cutter perpendicular to the direction of travel, and therefore the width of the stroke is always constant, which is the object of the exercise. And mounting the little magnifying glass on the trolley (so you don’t have to wear it) is such a good idea too. And then all of that ingenuity was swept away by computers.
    – Stephen F
    Commented Nov 2, 2015 at 15:20

Sometimes it's hard to imagine doing things without computers. Especially if you were born into a year when computers were ubiquitous. But somehow, some-way...people got around before them.

As fantasmagorical as it may seem...before computers, cartographers and graphic artists had ADEPTNESS and FACILITY with their hand tools. My own personal experience, for example, in this exact frame: I used an 000 rapidograph with a flexible rule to draw pseudo-parallel lines on town maps in the '80s. Sometimes it took several attempts before a "keeper" was arrived at. The lines would not be strictly parallel, but "parallel enough."


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