Line Weight: when to vary it and why?

In my art class this is a question that comes up a lot. It's a key aspect of many linear illustration traditions including comics art, as well as older forms of art that inform them. A lot of artists starting out wonder about what to use it for and when. And it common for the unorganized, haphazard accidental use of it to give our work a less polished feeling. That's not all good or bad, in my own work i like a bit of organic noise, messiness. That can in part come from some randomness in my line values. Sometimes I'm much more polished and clean. And part of that is making sure the reasons behind the line weights are very consistent and follow a specific set of style guides, rules. "If when, then this". but in a lot of my more personal or intimate work i relax that, to give it what i think is a bit more of an intimate feeling. I attempt here to go over some the basic foundational reasoning behind what line weight can do, and how it impacts the work. Let me know in the comments what you think! Also you'll find more examples of my own work, applying it in different ways here. http://www.salgoodsam.com/illustratio... In particular look at this examples for some very different approaches. Here, the contour line is very constant. it gives the hand a graphic frame, helping to pop it out of the white background. http://www.salgoodsam.com/wp-content/... Here i've used heavy raw brushy lines, to help give the figure a chunky but still expressive look http://www.salgoodsam.com/wp-content/... This one is an illustration for a neurology science journal, so i used a very clean line, with a fair bit of variation for elegance. http://www.salgoodsam.com/wp-content/... On this car, we have lost line highlights, finer lines in the distance for the front hood, and clean but brushy feeling to try to balance mechanical slick with a bit of warmth. http://www.salgoodsam.com/wp-content/... you can check out more of my art instruction via this site. http://dd.salgoodsam.com/ And please consider becoming a patron, it'll get you access to my comics, and for a few, even be a way to become a student!   / salgood