When people look at my drawings, their first question is often, “Do you use a ruler?” My ink drawings are straight vertical lines. Well, sort of straight vertical lines. Look closely at any of my drawings and you’ll see the lines are a little wavy, occasionally slanted, and sometimes they’re kinda kinky. But no, I do not use a ruler.
Drawings should flow. Lines should be free, varied, expressive and spontaneous. Even though I love the regularity of a fine pigma micron pen, I can still add variety to my lines. I can push hard, or barely graze the surface of the paper. I can draw fast or slow, precisely or loosely. Rulers don’t have a place in my drawings. They’re too mechanical. That being said, I recognize someone may have a style with rulers that works well. More power to them.
I thought about doing a small drawing with a ruler to show you the difference, but that would be tortuous. So instead, here’s a typical drawing.

And here’s some close ups of lines. I used fairly regular lines for the distant shore. Regular lines with no variation in value are good for indicating distance.
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On the side of the boat, there’s more variety in stroke and value

The one time I use rulers is when I teach perspective drawing. In fact, it was searching for rulers for my current class that prompted this post. When demonstrating the perspective of parallel sides to vanishing points, a ruler is essential. But other than that, it’s freehand all the way.