Line drawing — drawing without shading, cross-hatching or chiaroscuro — permits and conveys the most precise sense of accuracy of any kind of drawing. The facts are laid bare, nothing can be fudged or obscured. Leonardo used line drawing for his studies of everything from flying machines to the human nervous system.
The irony is that line drawing is less like the objects drawn than any other method of drawing. There are planes, shadows or colours in the thing we observe, but no lines. There are no lines in nature, in the human body, in fabricated objects. The lines in drawing are an invention. Line drawing is the most complete artifice in art.