how do you know which lines not to draw?

No.4693822 ViewReplyOriginalReport
how did the artist know that he should only draw one shoulder blade? how do you know which part of the elbow to make with a black outline, which to hatch ad which are best left to subtle shading. how do you know exactly how much of the collar bone to draw and how much to imply? there are no black outlines or crosshatching in real life.