These are the only two types of lines that you can use in drawing. So that means this isn’t that complicated, right? Well, yes and no. A line has a beginning, middle, and end to it. When the end changes direction sharply, it begins a new line. A line can end in open space as a hair on a drawing if that’s what you want it to represent or it can end in open space as an overlapping line with a tapered end to show dimension as in the examples below.
A curved line forms either an “S” curve or a “C” curve. This same principle is used in the “Line of Action” within your overall pose of your character. Avoid at all costs a “W” of “M” curve. They look awkward and in almost every circumstance destroy the “flow” of the pose.
When you’re doing your drawings, this is part of your thought process! Ask yourself, “What does this line represent? Is it an “S” or “C” curve? Is it forming a “W” or “M” curve? Does it look weird? Can I fix it to make it look better?, stronger?, more interesting?, more exaggerated?, etc.