When I first started learning how to draw I learned about cross hatching or hatching and thought it was a way to create a neat texture while drawing. I was completely unaware of it's extreme usefulness while drawing. To start, many artists from Peter Paul Rubens to Robert Longo use cross hatching in their drawings/sketches. Although there are different types of cross hatching that an artist can use.
Rubens was a very talented painter known for his use of brilliant reds within shadows. Many artists look to his paintings to learn about color theory and painting in general. But some go back and create copies of his sketches to learn how to render a figure well and learn basic knowledge of a figure. In this one sketch of Rubens you can see the marks are just straight lines. These lines are following the plane of each part of the skin to create darker areas as shadows. So where there are more lines there is more bends in the skin and more planes in which the skin uses. Get it?
Another few artists that use this technique very obviously and beautifully are Robert Longo, Jenny Saville and Simon Birch. This is a great skill to learn when learning about shading and rendering the figure. Many artists use this technique while copying masterpieces. Something to keep in mind while working with cross hatching or hatching is that the lines do not need to be straight, they can bend around the plane as well which allows for better rendering and following of the figure's skin. This allows a better 3 dimensional appearance to the viewer.
So if you're a new artist or an artist just needing some ideas, try cross hatching to further your skills in drawing.
Citations:
Rubens Sketch: http://www.metmuseum.org/toah/images/h2/h2_2002.12b.jpg
