Crosshatching is the layering of planes of parallel lines on top of each other in order to create a gradient or texture in a drawing.
Crosshatching has an "old-fashioned" stigma, probably for good reason: drawing lines side-by-side, and then on top of each other, is a great solution to a problem inherent in pen & ink drawing and printmaking: How do you make a drawing tonal if all you have to work with is black and white?
With digital tools at our disposal, as well as relatively new products like Zipotone, Craftint and DuoShade, it's easy to see why crosshatching isn't considered cutting edge. However, I don't personally believe that a technique in itself can be old-fashioned; I think that comes out of how the artist uses the technique.
Below is a primer on crosshatching for the beginner or for those who want to hone their craft. Professionals -- we'd love to hear your advanced tips and tricks in the comments!
Styles of Crosshatching
1) Tight, accurate lines. Plane 2 is arranged at 90 degrees, plane 3 at 45 degrees.
2) Organic lines. Gives a softer feel.
3) Fast, wild lines. Energetic and frantic.
4) Lines that follow the contour of the surface.
When an overlapping plane doesn't have much of directional shift from the plane below it, you can end up with an effect like below. It looks jarring and will take attention away from your drawing.
When drawing a 3D object like a skull, do you keep your lines straight or follow the contours of the object? Examples:
Two kinds of gradients below. On the left, the planes of lines end abruptly, in chunks. On the right, however, there is a greater attempt to smooth the transistion through the use of small lines that "dissolve" from one plane into the next.
There is no right way to crosshatch, but being consistent in whatever style you choose can go a long way towards making your work look competent.
Consistent line weight. Keeping a steady line is easier with an inflexible tip pen like a rapidograph. If you're using a flexible crow-quill nib, however, keeping steady line weight will be a chore.
However, consistency and a changing line weight can go hand in hand, but takes concentration: