Before computers, how did cartographers draw roads with perfectly parallel sides?

Question

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?

Answer 1

Calligraphy pens can be made with nibs that have multiple points which allow for perfectly parallel lines. You can still find nibs like this made today.

https://i.ebayimg.com/images/g/nlAAAOSwuxFY16~F/s-l1600.jpg