>>988411Well, the thing about lines is that they don't actually exist in nature. Edges exist in nature, so when you draw a line, what you're really trying to do is capture an edge. There are edges that meet with negative space, and edges that meet with other edges, and edges that taper off.
The reason certain edges are thicker than others (if the artist is competent) is because the artist has an understanding of light as well as form; edges that are facing away from light and taper away will reciever a darker, more solid line than those which meet with negative space facing light.
Take for example a plastic ball under a lamp.
The side away from the ball will be cast in shadow, and the edge of the ball will be more defined than the part of the ball closest to the light, which will be reflecting some of the light. The way to look at this is as a highlight, the line will dissolve because some of the "line" is being lost to the highlight.
In addition to understanding lines, there's how you put lines down. You should really be drawing on a large pad, but that option isn't always available. I do a few exercises personally before I begin really drawing, some of which include practicing parallel lines, circles, squares, and accuracy training, then maybe 10-15 mins of gestures and contour drawing or a negative space study. You want to loosen up your body so you can get away from making rigid marks.
Once the line is down (beginners always tend to "pet" their marks in), erase the excess marks and lightly erase the main line, then go back over it as smoothly as you can. Clean up the extra marks again.