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